Here's a case example. Build I helped a US friend with couple weeks ago:
XBOX One Controller
Samsung External DVD writer
Western Digital Desktop 1TB HDD
Samsung 850 EVO 250GB SSD
Intel i7-4790K Retail
Asus Z87 Gryphon
EVGA GTX 970 FTW 4GB
Logitech K400 Wireless Keyboard/Touchpad
G.Skill Sniper 16GB (2x8GB) 2133MHz RAM
EVGA G2 650W Gold
Coolermaster N200 Micro-Atx case
Windows 7 Home Premium
All new. And after taxes and rebates (which make up for taxes plenty), grand total of $1,200.
That equated to £775 at the exchange rate a couple of weeks ago. Can you fit all that hardware into an under £1,000 spec in the UK? Nah.
YOUR BASKET
1 x
EVGA GeForce GTX 970 FTW "Plus Edition" ACX 2.0 4096MB GDDR5 PCI-Express Graphics Card (04G-P4-3978-KR) £299.99
1 x
Intel Core i7-4790K 4.00GHz (Devil's Canyon) Socket LGA1150 Processor - Retail £289.99
1 x
Kingston HyperX Savage Red 16GB (2x8GB) DDR3 PC3-19200C11 2400MHz Dual Channel Kit (HX324C11SRK2/16) £79.99
1 x
Microsoft Windows 7 Home Premium SP1 64-Bit - OEM (GFC-02733) £79.99
1 x
Samsung 250GB 850 EVO SSD 2.5" SATA 6Gbps 32 Layer 3D V-NAND Solid State Drive (MZ-75E250B/EU) £77.99
1 x
EVGA Supernova G2 650W '80 Plus Gold' Modular Power Supply £74.99
1 x
Seagate 1TB SATA-6Gb 7200rpm 64MB (1 Year Warranty) £34.99
Total : £947.53 (includes shipping : £8.00 Ex.VAT).
Nearly up to £1,000 already and missing motherboard, XBOX controller, wireless keyboard/touchpad, external DVD writer and case.
Sales tax? Depends on state but in their case they paid sales tax (6.25%) on three items (HDD, SSD, CPU) so far. If they were to pay tax on the rest, it would amount to $48.06, or £30.79.
So once all is said and done, you get a build like that for £300-£400 cheaper in the US.