As any seasoned Brit will tell you, the old adage 'there's no bad weather, only bad clothes' is a survival mantra for the nation's damp, chilly winters. The secret to comfort lies not in a single thick coat, but in the foundational layers you wear beneath. When the mercury plummets, a good set of thermals becomes non-negotiable, trapping warm air close to your skin and wicking away moisture to keep you dry and cosy all day.
How We Put the Top Thermals to the Test
Our testing was driven by a personal quest for warmth without bulk. Having moved from a tropical climate, adapting to British winters meant finding base layers that provided serious insulation without feeling like a stuffed mattress under everyday clothing. We evaluated a range of thermal tops and leggings from high-street favourites and specialist brands, assessing them on several key criteria.
We wore each piece both indoors and outdoors, under regular clothes and under just a coat. A critical factor was breathability; we noted how much we sweated during brisk walks and how quickly the fabric wicked moisture away. We also judged how seamless and invisible each layer appeared under clothing, and of course, the fundamental measure of how warm and toasty they kept us from the moment we stepped outside.
The Best Thermal Underwear to Buy Now
1. Best Thermal Overall: Uniqlo HEATTECH Scoop Neck T-Shirt
Uniqlo's proprietary HEATTECH technology earns the top spot for its outstanding performance and versatility. This ultra-lightweight top feels like a second skin, generating warmth from your own body heat the instant you put it on. Its scoop neckline ensures it stays hidden under most outfits, and it comes in crew neck and turtleneck variations. While it retains heat impressively well, this can sometimes make cooling down a challenge during intense activity. At £14.90, it represents exceptional value, with testers reporting pieces that have lasted over five winters.
2. Best for Outdoor Activities: Peter Storm Women's Short-sleeve Thermal T-shirt
A rare find, this short-sleeved thermal from Go Outdoors is the ideal solution for active days when a vest isn't enough but a long sleeve is too much. The soft, close-fitting T-shirt style offers warmth without overheating, making it suitable to wear alone under a jacket during milder weather or physical exertion like hiking. Be aware that the polyester-viscose blend prioritises warmth over breathability and can add a slight bulk under clothes.
3. Best Fleece Thermals: M&S Heatgen™ Maximum Thermal Fleece Top
For the very coldest days when you want to feel like a 'life-sized teddy bear', M&S's fleecy thermal is the answer. Using their Heatgen technology, this top is supremely warm, soft, and cosy, perfect for lounging or braving bitter winds. The trade-off is significant bulk; it's thick and best worn under loose jumpers or on its own to avoid a Michelin Man silhouette. The matching leggings complete the ultra-snuggly set.
4. Best Lightweight Thermal: Intimissimi Boat Neck Modal Cashmere Ultralight Jumper
Beloved on social media, this top is a lesson in understated luxury. Made from breathable modal with a hint of cashmere, it feels incredibly soft against the skin and is practically invisible under clothing. It provides decent warmth for mild winter days but is not designed for sub-zero temperatures. The sheer, delicate fabric requires careful handling to avoid snags.
5. Best Silk Thermals: Jasmine Silk Pure Silk Thermal Top
This 100% mulberry silk top proves that luxury and performance can coexist. Silk is a naturally insulating and highly breathable material, making this top excellent at regulating temperature and wicking sweat, which is also beneficial for sensitive skin. It adds zero bulk and performs well under other layers in deep cold. The £45 price tag is an investment, and the delicate fabric demands careful maintenance.
6. Best for Lounging: Pour Moi Second Skin Thermal Long Sleeve Top
From the lingerie brand Pour Moi, this thermal set excels in comfort. The fabric is super soft, smooth, and stretchy, making it ideal for sensitive skin or relaxing at home. Available in a variety of colours with matching leggings (£25 for the top, £25 for the leggings), it creates a complete lounge set. It's not skin-tight, so it can add a slight feeling of thickness under daywear, but it excels in cosy indoor settings.
7. Best All-Weather Thermals: Smartwool Women's Classic All-Season Merino Base Layer
For the outdoors enthusiast, this Smartwool base layer is a standout. With an 88% merino wool composition, it offers superior breathability and excellent moisture-wicking, keeping you dry and comfortable during hikes or other physical activities. It performs reliably across seasons but comes with a premium price of £90.
Your Thermal Wear Questions Answered
Should thermals be tight or loose?
Thermals function by trapping a layer of warm air against your skin. Therefore, a snug, close-fitting cut is essential to prevent cold air from disrupting this insulating layer and to efficiently wick moisture. They should not be so tight that they restrict movement or cause discomfort.
What is the best material for thermals?
Traditional natural fibres like wool, silk, and cotton offer excellent breathability and temperature regulation, often requiring less frequent washing. Modern synthetic blends (polyester, acrylic) are very effective at retaining heat but can be less breathable and may hold odours. Many of the best options, like merino wool or HEATTECH fabrics, blend the benefits of both for optimal warmth, moisture management, and comfort.
The Final Verdict
Uniqlo's HEATTECH range remains the undisputed champion for everyday wear, offering unparalleled warmth-to-thickness ratio and lasting value. For ultimate cosiness, M&S's Heatgen fleece is unbeatable, while Intimissimi provides a whisper-thin layer for milder days. For active lifestyles, the technical fabrics from Smartwool and Peter Storm are purpose-built to keep you dry and moving comfortably, proving that with the right base layer, you really can conquer any weather Britain throws at you.
