Hivemaker,
I quite agree getting queens mated early in the season is so problematic in the UK. This is the reason why UK queens command a premium price over imported queens.
People contemplating early queen mating need to understand the relationship between when they see the first drones (on the comb) and first capped queen cells? This has a direct bearing on the earliest time of the year you could get qeens mated in your area.
I think it might be possible to get drones reared 2 maybee even 3 weeks earlier in the season but probably not much more. You'd have to have colonies in sheds and pamper them to get them up to strength.
If you did this you'd still come up against the main problem with the UK. As we are an island, the weather in April can be highly variable.
To get queens mated early you'd need to have a large number of colonies with drones in small area to get the drone population up to scratch and then you've still got the weather to contend with.
For example: my queens got mated in spurts this year and it was not unusual to have a queen in a nuc hive for 3-4 weeks before producing any eggs.
Just for info: I've not had any queens fail in the summer this year but I have found the odd colony with black queen cell virus.
But I have had a positive thought: As the pound has recently collapsed against the euro, the price of imported queens should go up which might help the queen rearing industry in the UK!!
