Depends what you mean by 'career' and what your current salary expectations are....The pay for F/O's in UK is very low.
If you are in your mid 30's you probably have a fairly reasonable income (?) and you'd probably have to go the self-improver route to get a flying job - so you have to consider is it worth;
a)forking out £40K - £50K in training for an ATPL and having to either not work whilst training or take significant time off work to train.
b)Loosing a few years salary and instead getting paid peanuts as a FI (no offence to FI's) whilst hourbuilding and trying to get a job.
c) Getting paid only £20K - £27K for at least the first few years.
d)having to spend alot time away from girlfiend/wife/kids and having to accept re-locationing, possibly several times.
Many people when they talk about 'how much it will cost' don't mention the (very large) cost of lost earnings whilst training, hourbuilding and early on in their airline career.
ie. If you are on a salary of £70K now, it means that by the time you have been a F/O for two years you will have lost financially;
50K in training costs
110K - 225K in lost earnings.
ie. worst case it could realistically cost you the best part of £250K !!
If you are 18 yrs old, never having earnt a wage before and without mortgage, wife, home etc then I guess the above doesn't apply, although the training costs are the same.
I'm not trying to put you off, but if you do fit into the profile of someone who has already a successfully established career, you really have to ask yourself are you prepared to slash and burn your finances, home and possibly familly...
Good luck!