BA introduced the fee in September 2007.....
There is a thread started on this subject every week or so. A quick search of key words will reveal the extent.

You mention BA was cheaper than United.
If you include the seat selection fee to your fare and it's still cheaper than the UA ticket was then perhaps this is one way to swallow the bitter pill and to justify the fee which is essentially an optional add on. Failing that some OneWorld status will do the trick - ruby at 7 days out, sapphire and emerald at time of booking. Generally it's not an problem to wait until T-24 and you'll still get a fairly wide selection to choose from. Despite having status my favourite seat in Club World only only frees up at that point anyway (basinette position rear facing window seat) so providing you're poysed to carry out online checkin as/when it opens, you should be ok.