We did this last year. If you're swapping out a gas fire for a woodburner, you will need to get a gas engineer to take out the gas fire and a stove installer to install the stove. Some people may be qualified to do both! If you're looking at the moment be prepared for waiting lists, a lot of the installers get booked up months ahead.
Worth getting some advice from an installer before actually choosing a stove as well, as there are lots of variables around size, shape, heating capacity, etc, etc.
Our neighbours had a stove installed very nicely by Ignite - you should be able to contact Richard 07793 321 101, we talked to him a few times and very helpful. We didn't use them ourselves simply because they were booked up for months! They will put in a stove and do any work around the hearth (laying hearth surface, plastering, mounting lintels, etc).
Richard (different Richard) at Stortford Chimneys did our installation - nice job, quick and efficient. He only did the actual stove installation, not any other work around the fireplace area. 07990 770 883
We got the stove itself from JDay stoneworks, they have a good selection so worth a visit along with Morley stoves in Ware, if you want to look at different models.