I can recommend www.pcspecialist.co.uk. You can design your own laptop or desktop. Just follow your nose through the website. They have a range of standard specifications that you can tailor to your requirements. They also have a very patient and helpful technical support desk which is based in Yorkshire rather than somewhere 5000 miles to the east.
I get most of my computer kit from Novatech.com.
They will make one to your specifications or they do plenty of ready to ship complete PCs at very competitive prices.
I have found all their equipment to be of a very high standard and they are very prompt in delivery of items.
First choose the computer you want. There are many on display at Currys, Comets or PC World. Then do a search on the internet for that model. You will be shown a list of that model, where to buy it and all the prices involved. Normally if you buy on line you may get a discount.