Joe,
That figure of £4000 PA was meant to be a broad brush example of the flight allowances that a typical LGW 757 (only) pilot might expect.
Again it is hard to say how many trips into work may be required in a month. For me it is 17 this month and a currently planned 14 next month. In Winter it is far less. The increasing number of year round scheduled flights will likely raise the low season average.
Seasonal working of the type you descibe is not currently offered.
There are staff travel concessions with other airlines subject to service requirements. You normally have to have been continously employed for 12 months (6 in a few cases), however it is normally subject to individual applications and if it is for the purpose of commuting almost certainly a non-starter.