I forgot to mention that within the engagement freeze you can bid for a same seat/same aircraft , i.e. FO on the Airbus at LGW to FO on the Airbus at LHR, transfer to LHR which should be processed in seniority order on the annual bid and attracts no further freeze. So, given that recruitment is to both bases you should be able to move to LHR is you desperate to be be there, certainly within a couple of years.