No qualification is mandatory for electrical work, all you need to do is to prove competenancy, so therefore a qualification proves that. If you have the 2391-10 then I would be amazed if anyone questioned your competenancy even on PAT testing, but for the sake of 300 pounds it may well be worth doing the 2377-002 as it will give you a better insight into PAT testing.
On a personal note I only did PAT testing for clients who 1 asked me as a favour to them, and 2 If I wanted to win a contract or a job it was another service to offer. For a single operator it is very hard to break into as was mentioned before as larger companies offer testing at 50p and below.
As you have the 2391-10 then why not think about PIR's. They are a better option for earning a half decent living as a small house can be costed at about 125 pounds, espicially as your a single operator with little overheads. If you go in cheap to an letting agent and guarantee a low price for a year they may sign you up. If you do 3 a week it's nearly 400 pounds, also you do the PAT for them, could be a way to start