I would say 9/10 people doing foreigners use their bosses stock. It is so easy to do though. I installed a full central heating system in a house I lived in and all the fittings, rad valves, clips, gas, solder etc was provided by the company I worked for. (I didn't even pay for the radiators but that was not through the company)
That to me is why anybody doing a foreigner would be gone in a shot.
I understand that there are people out there that are setting up for themselves who do work evenings and weekends and have all the costs but most of them still take stock off the bosses.
The old boy that taught me used to do a boiler every weekend (this was before you had to register them) and he used to build his stock up during the week then use it at the weekend. The boss found out because as mike said a customer took the number off the side of the van and couldn't get hold of him so phoned the company. That then made the boss stop all the lads using the vans outside of work and he got a warning (he was never going to get rid of him as he was the only gas installer they had)