PHR street kit is $3695. The interbank exchange rate today is 1.55 which means you will probably get 1.5. That makes this kit £2463. Say shipping is $200 (minimum) that is another £133. So we are at £2596. You will probably get away with just paying VAT so you are now at £3115.20. Not sure what the exact duty rate is for these parts but will be at least 2%. That would take it to £3167.
Ok, so this is £800 less than the cheapest complete Whifbitz kit, but I know which one I would buy. I hate to think what the aftersales service would be like if bought in the US, shipping parts back there for inspection and shipping replacements or repairs back to the UK afterwards. The £800 saving could disappear very, very quickly!
But each to their own.