Personally I would far prefer to do a job I loved for crap money than tolerate a job I don't like for better money. Sometimes there isn't the choice I realise, but I do believe it's important to do something that gives you real satisfaction if at all possible. That satisfaction is "payment" of a different kind iykwim. If you can do a bit of both so that you don't suffer too much financially, so much the better!
Answering to your original question, er.. just if the dream job is paying a realistic salary for the position. For example my ideal job is within design, many places offer ludicrous salaries, would I work for them? NO, once the novelty starts to wear up I would feel they are owing me something. However, if that low salary is adequate for that position, and I'm unlikely to get bored of that position within some short time then is fair and I would do it.
But before leaving a well paid job for a dream one check if you really want to let go of the things (expensive holidays, clothes, going outs, etc.) that that high salary allows you to enjoy.
I was working part time, and earning around £450-£500 a month, sometimes I was taxed cos I just went over the threshold, but it was minute amount.
One time it was like, 67p NI
I don't know the actual cut off, but I am sure it's online somewhere!
You will pay more tax on a second job unfortunately though!
You can specify which is your main job, where you want your personal tax allowances (£4745 till April), then you pay basic rate (22%) on all your earnings at your second job. (Unless your total income takes you into the 40% bracket..)
If you want your new job counted as your main job you need to tell your tax office so they can sort it out. HTH.