Short answer: new, unworn underwear with tags is usually fine. Worn underwear generally isn't, for hygiene reasons — this is standard policy across most major UK charity retailers, including British Heart Foundation and Oxfam, not a fussy local rule.
Most charity shops cannot resell worn underwear no matter how clean it is. If it's new and still has tags on, it's generally welcome. If it's worn, your best options are a specialist scheme like Smalls For All (which collects new pants and gently-worn bras for redistribution), or asking your local shop if they accept worn textiles as "rag" for recycling — many do, even though they can't put it on the shelf.
Slightly more flexible than underwear: unworn or lightly-worn bras in good condition are often accepted for resale. Heavily worn bras usually aren't resold but can often still go to textile recycling, or to a bra-specific scheme like Against Breast Cancer's bra banks.
British Heart Foundation's own donation policy states it clearly: worn swimwear without an intact hygiene strip cannot be resold. If the strip is present and clean, or the swimwear is unworn, it's generally fine.
This isn't charities being precious — it's a genuine, sector-wide hygiene standard, since these items can't be sanitised to a resaleable condition the way outerwear can. It also means you don't need to feel awkward asking a shop directly — staff answer this question constantly and would rather you ask than end up disappointed at the till.
Check our full list of items charity shops typically can't accept for the wider picture, or use the directory search — searching any item automatically flags anything with special conditions like this.