There are Three schemes operating in the UK. The Refrigeration / Air conditioning company you use legally has to be registered with one of the following bodies:
The F-gas Register (powered and run by)
With Refcom and the F-gas resgister, the end user (you!), is able to search for companies on their corresponding web sites. For example: AACR has chosen to be registered with Refcom. One would go to the Refcom site, look for the panel that has a button "scheme members", and click on it, and then search for AACR.
The F-gas resister search is a little easier, in that the "company search" is directly from the home page.
With Bureau Veritas, this facility isn't available: You will have to contact Bureau Veritas to check.
In all cases, some companies may have chosen not to "go public" with their registration, in which case, if you have any doubts, you'd be wise to ask the Environment Agency F-gas support team for verification.
Remember, it isn't only companies that are not registered that are breaking the law: As from 1st January 2015, YOU are too if you are using them!