when you have a vasectomy, on of the questions they ask is about remarriage, death of an existing child, etc. so you do accept that it probably isn't going to be possible to reverse it when you sign up. i think it's a bit unfair to then criticise the nhs for something you/he accepted quite happily at the time.
my advice would be to pay the �2000, there really doesn't seem to be an alternative, and if you're both so keen to have a child together it seems a small price to pay.
and also to bear in mind that not all reversals are successful, so there may not be the happy outcome you want even if you do spend the money.
sorry to sound negative, but you have to be realistic.