the reason for the queues is that everyone goes when they can - ie lunchtime, which is the time when there are the least number of staff (because they're on lunch) - so it's obvious that if you go when everyone else is going, there will be a queue.
I use the post office about five or six times a year - for things like currency exchange without commision, DVLA forms, and passport forms etc... occasionally I buy stamps there, and once a year send my uncle in Canada a christmas present/card.
I don't think I'd be inconvenienced if some of the smaller ones closed down.
A possible solution could be franchise in/near to a supermarket, with the caveat that the supermarket didn't supply the same services as the PO - ie not selling stamps, or having a post box - that kind of thing. These would be in places where lots of people go anyway, and would be subsidised by the large supermarkets.
queues - staff are costly - a subposmaster as a small businessperson has to think carefully about employing an extra person. It would have to ensure that extra business was brought in. However, the PO on Aldwych is one of the 500 or so POs which are publicly owned and so costs there are much higher and making a profit is less of an incentive (because such offices don't make a profit - though they could in private hands - the one on Westminster Bridge road is an example of this). In SE1 the two on Borough HS and the one on Blackfriars are publicly owned.
Going in with supermarkets - this has been tried with mixed results. Supermarkets keep a line on if it makes a certain level of profit per square foot. POs tend not to meet the targets. I remember two example of POs in SMs being closed and replaced one with a roast chicken counter and the other with a pizza section. So when I worked in this area it was a big worry when the supermarkets started buying up small conveneince store chains in which POs were located. I could see this leading to closures - though I am not aware of this happening yet.
>>at the Aldwych there must be sufficient demand that an extra member of staff would lead to increased sales that would more than compensate
It's not really increased sales that determin whether or not an additional member of staff can be afforded, it's profit. Unfortunately, I guess, that the profit generated per item of sale in a post office is just too low. Put the price of a 2nd class stamp up to 25p and and 1st to 38p and it may be a different story