This might surprise you, but if your household income (thats two combined incomes or one if only one works) is £35k after tax, then you are withing the top 10% of earners.
People who earn more than £50k are a negligible proportion of the population.
The main flaw with the proposal is that the majority of the people who received a lump sum of £20K would just stick it in bank account and save it, put aside for college fees or some other rainy day strategy. A large proportion of the hand out wouldn't be circulating so it would have very little effect.
Hand outs and tax breaks to the well off don't work, as the well off don't spend all they have from one month to the next, so they'll just have more surplus to put by for a rainy day, and when that rainy day comes they'll just emigrate to Spain, mumbling about immigrants under their breath. (They will if they listen to Jeremy Vine anyway)
Handouts / breaks to the the poorest have the most benefit to the wider economy as the poor inevitably spend all the money they have, so with things like the VAT cut might only put a few extra quid in most people's pockets, but they'll spend those few extra quid. And there are many many more poor people than rich people.