N_D,
In a nutshell, the career progression for a pilot in the RAF might go something like this:
Flying training, OCU, front-line tour. At the end of the first tour, he/she will in many cases stay on the same aircraft type for a second tour, but where feasible moving to another base (or perhaps another Sqn on the same base). This benefits both the individual (who, by staying on the same type, can build on their experience and work towards an Above Average rating, necessary for promotion or instructing) and the RAF, who have a still young but now reasonably experienced operator to help those arriving from training.
After the second tour, it's usually time to do something different. This usually means either promotion or becoming an instructor. After this third tour, many people either work for further promotion (which almost inevitably means a ground tour or two), or lose themselves in the world of flying training, or leave, having done by now at least 10 years' service (including their own training). Some lucky people may get an exchange tour (ie flying an equivalent aircraft type in a foreign air force), or perhaps change aircraft type (Herc to C-17 as one of many possible examples, also Tornado to Typhoon etc etc)
This is a huge generalisation, but it does apply to a great many people. Two things to bear in mind: as far as I know (but I'm sure I may be corrected!), what I've written applies more or less to FJ and RW and ME pilots alike; also, once a pilot finishes his/her training, he/she will in almost all cases stay in that stream. Plenty of people are keen to change stream (eg ME to FJ), but not many people manage it. Getting into or out of RW is particularly rare due to the amount of re-training required - but then it does happen for some people.
Hope this is of some use,
TOTD