If you're ordering anything into the UK, it's a bit confusing as of January 2021, so I'm sorry for the wall of information coming your way, but the short answer is that you *should* be fine.
Long answer:
When you import into the UK, customs assess how much you owe in VAT based on the customs declaration form stuck to the package. If you're importing books, which include doujinshi, as long as they're called "books" on the customs declaration, you won't pay any VAT as books are "zero-rated". If you were to order other items, those may get VAT charged, which is 20% (same as buying things in shops in the UK) and then the courier will charge extra for dealing with customs on your behalf. VAT does vary (some items other than books are zero-rated or exempt, while others are at reduced rates, check the HMRC website for this).
As of January 2021, the UK have said that companies selling to the UK remotely or *facilitating* the sale of goods to the UK (such as a proxy service) are supposed to take VAT at the point of sale. This means the company needs to be registered for collecting VAT and thus file paperwork to the UK to report VAT collection and pay that money accordingly. I contacted HMRC to ask about what happens if the company abroad is only sending books (such as a proxy only sending doujinshi). In these cases, HMRC say the company would still need to register for VAT, but file for an exemption that they're not going to be collecting any since they only handle zero-rated and/or exempt goods. I might have misunderstood them, or got something wrong, so please don't take this as financial or legal advice.
OBVIOUSLY, many businesses are not doing this, and COVID-19 has meant that everywhere is understaffed and/or overworked, especially when it comes to courier services. As such, Border Force and HMRC are seemingly letting things sent via standard post services* (handled by Royal Mail and Parcelforce in the UK) through without complaining about VAT not being taken at point of sale. HMRC refused to say what would happen with parcels from companies that aren't VAT registered, and there's nothing on the website to say either. You'd think they'd start rejecting parcels or something, but there's nothing saying that.
Right now (8th April 2021) you are probably fine to import whatever from wherever because nobody's really sure what's going on and, particularly if you're ordering from very small companies and indie artists, it's likely too much hassle to chase them down over tiny amounts of VAT.
So yes, import away, and thank you for supporting independent (doujin) artists and groups.
*I say "standard post services" as I've had stuff delivered by DHL recently, and they have been charging VAT and handling fees like nothing has changed.
