only thing is the shipping time, it can be anything from 10 days to a month. Though I find sales from Chinese ebay sellers arrives quicker then buying from Chinese online retailers (dealextreme for example).
You won't face any additional charges unless your order is worth over £15, in which case you'll pay import VAT at 20% on the item. If it's over £135 you get customs duty slapped on as well, though if your custom duty was going to cost £9 or less, they'll wave it anyway.
When it comes to ordering from China though, even big companies, let alone ebay sellers, often lie about the value of the product when filling out the declaration. I've ordered quite a few things and say I've paid £10, which wouldn't have been taxed anyway, I'll notice that the seller will claim the value in HK dollars(lots of places based in Hong Kong) is say 45 or 90, which is actually much less then what I paid in pounds. Also most say it's a gift, which ups the threshold of import VAT to items valued something like £40 or more.
Basically they do it to make themselves more popular, people like not having to pay tax to put it bluntly. You won't get in trouble for that, HMRC aren't exactly bothered about little Chinese gadgets and fake jewellery which might be a little undervalued, they have much bigger fish to fry.