OK serenequeen, here are my thoughts for what they are worth.
From what I know if faith schools, I believe they are allowed to set their own entrance criteria, so if attendance at their preschool is important then that's it. But check anyway.
I think you need to look to the future. If this is the school for your son, then let him join the school's nursery. I think you might be pleasantly surprised how much he enjoys the change. Both my sons had to leave lovely childminders and nurseries they were happy in betweeen the ages of 1 - 4 years (twice in each case). When we moveed house or our working patten changed we just had to make adaptions to their routine.
Of course they missed their old place and faces, but not half as much as I feared. They talked about the past from time to time, but not in a very sad or tearful way. As long as the new place or person was a good one and I showed I was positive about the move my sons were fine. I was I who was the more upset and nostalgic.Anyway, even if your son stays put at his old nursery, he will be leaving it soon, when he starts school.
I agree with others who suggest a childminder. It really is a common solution and IME a good one. And your son will start to mingle with children from the school( if the childminder looks after others) so when he joins he will have some older friends there.
You have to decide whether A) you want a childminder to pick up from one nursery and drop off at the other. Or B) you want a childminder to look after your son for the half day he is not at nursery. Sorry if I am repeating what has been said. Personally I'd go for B). I think going to 2 nurseries, plus having a childminder is a lot to cram into your son's day. The excitement of meeting all those different groups of children might just be too much.
I know you feel the 2.5 hours routine will tie you down while you are on maternity leave. A possible solution is to get a childminder to pick up your son from nursery for say, a couple of days a week. Even if she just has him for 2 or 3 hours, this will give you more flexibilty on those days and it will get your son used to a childminder before you go back to work and he has a childminder every day. Mind you, finding a childminder who can do part time now and full time later on might be difficult. But not impossible - worth checking out. TBH if it were me, knowing how easily my sons slip into different routines, I would consider getting one childminder part time while I was an SAHM and then if necessary changing to a five day a week childminder who could also take my baby when I go back to work.
A lot depends on the availability of childminders in your area. If they are really hard to come by, things will be harder. A nanny might be the better solution.
Now, childcare vouchers. In my part of SE London all parents of children aged 3 and over get 2.5 hours free nursery time per day. If this is the case where you live, this saving would offset some of the cost of a nanny if you have to employ one.
I can understand your annoyance at the 2.5 hour arrangement. It is the tip of the iceberg, I'm afraid. IME many playclub and group care arrangements times are not convenient for full time working parents. For the past two weeks my oldest son has been attending a cricket course. It starts at 9.30 and ends at 3.30. Most sports courses have these sort of hours or less. I am lucky. I work locally and can drop ds off (am) and take a late lunchhour and collect him (pm) before driving him round to a neighbour. If I didn't work locally and didn't have a family friendly working environment I'd be totally stuck!