It is possible to buy a basic new fork for about £20 to £30. eg Raleigh 700C 1 1/8" Threaded Fork from Evans. Or there's a variety of options on Ebay, either new or used. Or some cycling forums have classified sections, so you could post a wanted advert.
Though there are several things to check to ensure a replacement fork is compatible:
- wheel size. That Ridgeback has 700C wheels, so you need a fork the right length for those.
- brake mounts. To use your existing brakes, you need mounts for V-brakes.
- steerer tube diameter and headset type. I can't find the details for that Ridgeback, but from photos it looks like it uses a threaded headset. It probably needs a 1 1/8" steerer, though it is possible it uses the older 1" size.
You could find a bike shop to fit the replacement fork. It may cost anything between £20 and £50, depends on the shop and just what needs done.
Or if you want to replace the fork yourself, you will need several tools. Probably including several spanners, and possibly also headset tools. If you don't have these, you may be able to borrow them from someone. The best option could be a DIY bike workshop, eg the one suggested by Booker. They will have tools you can use, plus they may be able to advise on what you need to do.
Depending on how your fork was damaged, its possible that other parts of the bike are broken. So it is worth checking it all carefully. The worst would be if the frame is bent or broken, that would be a lot of cost and work to replace. If the frame is damaged, you may be better off just replacing the whole bike.