Warranty is a pretty straightforward matter if you buy from overseas - year 1 you send it back to base and pay the postage (I priced that at about 40gbp to Hong Kong with a courier) which would no doubt be cheaper if you had a UK lens. Outside of warranty it's the same situation regardless of where your lens is from. You will have to repair it locally in which case the price is identical. So it's a trade off between a slight expense should something go wrong (unlikely) vs. considerably cheaper gear.
As for the legality of it all, getting anything over 390 from overseas ought to be declared for assessment and if you didn't declare you've broken the law already (and willfully, ignorance of that isn't an excuse as they say!). Next time you go abroad and bring back some clothes and souvenirs etc. just bear in mind anything over 390 needs declaring
I also think the risk that you will receive a knock on the door and be asked to produce all your receipts is unlikely in the extreme. The choice is a personal one I guess, as is the risk-appetite, but what's to stop you saying you bought it overseas on holiday for 385 and don't have a receipt? Where does the burden of proof lie? I think it's exactly that reason you don't find people talking about their experiences of being hounded by HMRC for buying the odd item for cheaper abroad, or on buying from abroad on the likes of ebay. If you've ever used ebay much you will notice that the overwhelming tendency is to underdeclare (or not declare at all) the value of the item - HMRC will no doubt be well aware of this but enforcing the millions of cross border sales annually would be simply impossible. It's disproportionately costly to go for one person buying a lens or two for personal use who still pays a huge amount more of UK VAT anyway (just by being resident) vs. someone bringing in a large quantity of goods to sell for significant profit. Both are as "legal" as the other but one has a significantly higher impact on the economy & a return for HMRC than the other.