These are my personal views and are given without any guarantee that they are appropriate for your situation.
Where to buy my new computer?
Occasionally high street retailers have really good deals on computers - but these are few and far between and are often special purchases or clearing out end of lines. For the best deals it is nearly always better to go to one of the big specialist online retailers. When comparing across retailers, particularly with laptops, take care that the model numbers match as a slight difference in the model number may mean a completely different processor or hard drive specification.
With the demise of Misco in 2017 I don't really have a recommendation for an online retailer, I use trade only retailers so can supply most things on request.
As an alternative look at Morgan Computers (http://www.morgancomputers.co.uk/) - they often sell 'last years' models from the big vendors and also sell refurbished kit. There are often very good deals available on refurbished laptops but stock availability can change from day to day.
What to buy (Vendors)
The 'big 3' computer vendors are Toshiba, HP/Compaq and Lenovo.
Toshiba are best known for laptops, although they did supply desktops for a while as far as I know they now purely sell laptops. Their business ranges would be my first choice for a laptop.
HP/Compaq have a rock solid reputation in the business desktop and server markets and their business laptops are also very good.
Lenovo are a Chinese company who took over the IBM PC business - they aim at the business market across all sectors and are also a good option.
Other computer vendors are available...