Initially commuting for shorthaul will be very hard work,as by definition you will have little relative seniority on whichever fleet you go to. With time things will improve as others join below you.
This is true for the Airbus A319/320/321 at LHR and the B737 at LGW. It may not be true of the B757/767 LHR if you are lucky enough to start on this fleet, as it is very popular and some people transfer to this fleet once out of their initial (5 year) freeze, meaning you have to wait a long time to gain relative seniority.
Most shorthaul trips start early (too early to commute in by air) and finish midday/mid afternoon, or start midday/mid afternoon and finish late (too late to commute back home by air), and frequently blocks of work start early and finish late, making it worse, so overseas commuting although done is not that common, and would be very difficult IMHO in the early days until you have at least some bidding control over your roster.
As for the current pension provision for new entrants, it is very poor.