In reply to Darkskys:
I suppose part of it is... do you want a vocational career or do you just want something which interests you and hopefully pays well?
As I asked above, what skills/exams do you already have? Also, more importantly, what does the inner you want to do for a job or at least what type of job?
Someone mentioned accountancy and I'll comment on that as it's what I know. If you were to do a pure accountancy qualification, it would mean several years of hard studying whilst working. However, people do get into it fairly late on. If you go the traditional route, you will end up doing some drudge work early on (audit junior...) but within a fairly short period of time you can start doing some pretty interesting work and specialise eg. corporate finance, business restructuring, tax etc. Many people stay in accountancy practises, some go into industry (as internal accounants), whilst others use it as a springboard into general business.
Others specialise in something like tax from the start which can be done without going the formal accountancy route but by doing separate (quite hard) tax qualifications, again whilst working over a number of years. However, if you fancy something which uses your brain and is actually quite close to law (without the fuss of retraining as a lawyer), it does have something to commend it. I found it an enjoyable career and it's possible and normal to further specialise. Although it might not sound it, it's also a very client focussed business with lots of client interaction and, if you work on the private client side of things, you will end up spending more time understanding and working with your clients than doing pure technical work. Regardless, tax is not all about figures, very far from it. Again, you can remain within an existing accountancy practice, go into industry (internal tax manager at a large corporate for example) or even set up your own practice in time.
In terms of quality of life, the early years would be intensive, but pay rapidly increases to compensate. After a few years you can start to get your life back. In the very large accountancy practices it may never be a 9-5 job (work hard, play hard mentality) but you can find a balance.
Of course, to get a training contract (where you work for a firm and they pay for your training over several years - a not inconsiderable cost), you will need to have good exam results so far and demonstrate commitment etc. You don't say if you have a degree or whatever. If you fail your exams persistently, you will end up being shown the door due to the terms of the training contract. As the tax exams can be done over as long a period as you like, that might be an easier route in if it appealed in any way.
The pay can be good - from ok to almost as much as you can imagine (fancy being an equity partner in the Big 4 and earning around £1m each year?). However, don't underestimate how hard it is to get used to working whilst studying and doing exams, there is the risk you don't pass the exams and end up having to retrain again, and whilst not a vocation it is rarely a 9-5 job even if it is office based.