Tax and National Insurance on £37 500 pa is approx £10 500 so you will clear £27 000 p.a. before any pension contributions. You will probably take home about £200 per month extra in allowances if you don't rip the ring out downroute on every trip. (That's on the 340 fleet.)
AT LEAST 5 years in the RHS, probably more like 8 yrs at the moment.
Once you have 6000 hours (believe me after 5 years in the company that won't be a problem!), command assessment is on seniority - that doesn't mean you'll automatically be given a command, just that you will get a shot at it. There have been 1 or 2 instances of people being promoted out of seniority, but they are VERY much the exception and were always for clearly defined reasons.