Any recommendations on choosing builders?

Hello @diydaddy. Welcome to Scope. 

One way of finding a builder is to ask around for names and recommendations. When I was in need of a builder, I asked on Facebook. Or you could always look at websites where tradesmen advertise their services as well.
This is a tool which you can use to find a builder. 
https://www.fmb.org.uk/find-a-builder/
Make sure to trust whoever you use additionally. Read reviews published online. Here is a list of useful safety tips on hiring builders. https://www.fmb.org.uk/find-a-builder/before-you-start/keep-away-from-the-cowboys/
Obtain planning permission from your local council. Always have a contract and get at least two quotations in writing so that you can compare prices carefully. Do your research properly beforehand too.