Every now and then ATS, quickfit or such have an a/c offer on. I had mine done last year for £80 with the correct gas.
It's not exactly labour intensive, they connect a machine up to your inlet/ exhaust valves, press the button and the machine goes through a sequence of removing your old gas, vacuuming and refilling. The correct 460g weight of gas required is written on the slam panel to the left of the bonnet latch.
The "technician" does not even have to be present through the procedure, the machine shows a completed/ succesful message and beeps when it's done.
Mine worked fine, before and after. The only issue you may have is if there is a problem with your a/c holding a vacuum, this is tested first, if it does not, they tend not to proceed and don't charge.
Just to add, they suggest having your a/c mainatined and refreshed as there is a lubricant in the gas (?) that helps maintain the compressor and seals. It also tells you the health of your a/c system, if your gas weight is found to be considerably down on the required 460g, you may have a leak problem?