Hello Niphawan and welcome.
I don't have a specific list of installers in your area but Bristol does have a few support options for people considering EWI.
May I suggest visiting either the Warm up Bristol website HERE. They also have a specific page for EWI HERE. They state on their website that the cost of EWI for a typical semi-detached house can start from as little as £6k which appears very reasonable but I don't know how up to date that information is.
You can also visit The Centre For Sustainable energy Bristol, who can also point you in the right direction of choosing an installer. They can be contacted on 0800 082 2234.
Finally, you can also visit the Bristol Green Doors website who can help with sustainable solutions of insulation.
Please let us know how you get on.