The £5000 (pre-tax) bounty is paid on completion of submarine training (on average 12-14 months after joining the service).
If you opt to be a submariner, regardless of bounty paid, you cannot change to general service (surface ships) until you've served 5 years on boats. Most who join as submariners tend to stay submariners as it's a good wage (£300 per month on top of the flat rate of pay to start with) and most guys take on the financial commitments relative to their income as they go through life.