You can take any of the routes already mentioned, or you can pay one of the companies that specialise in providing portfolio evidence. You pay them around 1K for the privilege, but if you're working for someone else, and not being paid it could mean that it ends up costing you more. Depends on how long you work for free I suppose - and yes, I do know that you get paid in knowledge but as they say, time is money and every day you spend with a Gas Safe registered engineer that isn't paying you (Monetary value) could be costing you £200 or more depending on your hourly rate.
I'm not saying that either way is better, just that it's a kind of 'Horses for courses' thing in the end.
Being a registered OFTEC engineer with 26 years experience I already have a decent amount of knowledge but I still have to go through the process so I suppose in the end it all depends on what you want to get from the 'Evidence'.