Items bought from overseas / outside of the EU and posted to you have a £15 allowance. For anything worth more than that (excluding the postage) you pay import VAT, which will be 20% of the item value.
For a gift the allowance is £35
(Note that 'gift' means something that an individual has sent to you - a parcel from a Hong Kong airsoft shop marked 'gift - value $0.01' with your real receipt inside does not qualify as a gift and is customs fraud! Legally you as the recipient could be held liable for for fraud, however customs know that this is pretty much the default for many Hong Kong imports and quite often they would be exempt from import vat or duty any way.
For items over £135 both import VAT and import duty could be payable, but if the duty is below £9 it won't be charged for
The majority of time for items imported by mail customs don't get involved, the main couriers are licenced and manage the majority of packages passing through their international hubs, they will tend to handle it all themselves - send you the bill and then pay your import vat/duty to HMRC. This is why there is a typical handling charge of £8 for it sitting in their depot, being administered and handling your payments
When importing by mail, generally assume there will be 20% + £8 to pay, you might not get charged but at least you've taken it into account in advance
Unless the item is unique, then ideally support U.K. retailers. Even if it is unique then a U.K. retailer will probably be willing to do the import for you - they get some business and they may pay less overall for postage and duty, the duty gets offset against vat (may or may not make a difference) But you could make a saving due to trade prices only paying for their profit.
When you are abroad and buy items to take back then the allowance is £390 (Bear in mind the actual price you pay while in America may be more than the price shown on display as they add local taxes when you pay, as opposed to here in the U.K. where prices shown are inclusive of vat)
** of course - check the HMRC website for the current rates and allowances, (the above are the allowances I have been aware of - and can also vary depending on customs category), ensure you can carry the items on the plane (whether in hand luggage or hold luggage) and obviously make sure it is legal to import / you have the appropriate information and documentation
The value used for the calculation of import VAT and import duty is not necessarily the price you paid - HMRC can select the value that they will use which could be the U.K. retail price. (This is more likely to occur if you attempt to sneak them in without declaring)
If you take valuables or equipment out with you that could flag up on return then make sure it is documented that you took them on your way out
