boyoneder,
no, not just senior cabin crew do longhaul flying. this is how it works:
if successful, you will be recruited onto the shorthaul fleet at LHR.
then, later on this year - around Oct/Nov - The Opportunities and Choice Framework will open again. this opens once every two years and it enables you to put your name on a transfer list to transfer to a different fleet (i.e. longhaul LHR; singlefleet LGW etc) and to bid for a change in contract type (i.e. to go from 100% full time to either 75% or 50% parttime).
usually, you have to have done 2 years with the company before you can put your name on any of these lists (however, last time round they scrapped this rule at the insistance of our union, BASSA, due to there being not enough crew with that level of seniority to fill the lists sufficiently. so it was open to all crew no matter how long you had been with the company).
then the lists are compiled into seniority order within grade. after that, you just have to wait for a phonecall/crewlink message offering you a transfer of parttime.
however, i warn you that you must expect to wait AT LEAST 2/3 years for a transfer to longhaul from shorthaul, and around 7/8 years for parttime.