There are many companies in the UK that can supply & fit a new gas boiler but to get some good information you should visit a guide for boiler replacement from MLP
This website covers the different types of boilers that are in the market and offers the opportunity to take a look at the advantages or disadvantages of each – very useful if you are not to sure about what type of boiler to get in the first instance.
Basically you can get smaller “instant hot water” types of products or the more substantial ( & complex) system type boiler. It may even be that you don’t know which type you have fitted in your property and all this means very little to you.
As a quick guide, if your water heater is fitted to the wall in the kitchen (maybe inside a cupboard) and you have trouble running a hot bath, shower & using the hot water in the kitchen at the same time, you could have a combi boiler. These are best suited to properties that don’t have high simultaneous demands for hot water.
If you have a big insulated hot water tank in an airing cupboard, then its more likely that you have a system boiler – of the 2 types, this would be the more costly to replace.