That's a no brainer, you will drown on your own trying to fix boilers, at best you might get some customers to pay for the parts that you chuck at the boiler to get it going, but your reputation will take a hammering by visiting 3-4 times and invoicing £300-400 to fix a boiler, at worst they won't pay you for the first 2-3 visits as it isn't working, they will know you are bluffing/struggling.
If you get a job with a firm you want to be on servicing for a while till you get a feel for working on boilers, the ideal situation, and one I used myself for 6 guys we took on via gas foundation where they were retraining at college and coming to us for placement was to start them off servicing only for 6months, then as they got better I put them into 2nd fix repairs, we looked after 14,000 HA houses that got serviced every 10 months, plus repairs so we had the contracts for this, if one of the regular guys needed to order a part and go back cause it wasn't part of his stock I would send one of the new guys to fit the part, it gave them experience and confidence of fixing, and if the first part didn't fix it they would phone the original guy and talk to him about what to order next, once their experience built up they got onto trying first fix to see how they got on, it worked well, and the 7 of them are still there and have done their ACS renewal so that's 5 years under their belt now, I appreciate these jobs are few and far between but go for the contractors who are doing the HA work you will get maybe £10 per hour to start with but with limited experience it's all you will get, and then it's up to youm to prove you are worth more