If you are concerned about the cost then you will find the cheapest prices online. Choose a reputable website and check out their warranty terms, return-to-base (RTB) or on-site support options and delivery costs before you place an order.  Remember, if the support is RTB you may have to send the computer back by courier at your cost, leaving your computer (and probably your data) in their hands for some time.

Some time ago I ordered an HP PC via the John Lewis website for someone.  That was relatively straightforward and easy, which I mention just to show that there are some high street retailers who also offer an online option. Read the online terms carefully.

If you prefer to be able to take your computer to a shop, then you will know as well as I do the electrical retailers who sell computers.

Just be aware that the advice you get in those shops may not be 100% accurate from what I’ve heard from people, so do your research before going in. If you’re not sure about what you’re told, go home and check it out rather than being rushed into a purchase. The prices of computers either fall, or remain the same while the power/speed increases (this has been true for decades), so don’t worry too much about missing a “bargain”!