As previously pointed out, the vast majority (if not all) of these suppliers are not UK based, even the ones with .co.uk web addresses & don't have their own UK warehouses. They use holding facilities & `middlemen` & rely on your purchase not being stopped to avoid VAT etc. hence being unable to supply a VAT receipt. Some also mis-declare the value of goods.
YOU as the purchaser are officially the importer & so responsible for any duty etc.
Most manufacturers won't entertain `grey` imports for warranty claims (+ the manufacturers warranty can sometimes be for 2 or more years, as opposed to the suppliers less comprehensive 1 yr)
Use caution when offered an extra couple of % to pay by bank transfer. Paying
anyone by direct bank transfer is absolutely not a good idea, even to save a few extra quid.
imho, peace of mind is worth a fortune.