I honestly can't believe how judgmental this thread is. I'm a single parent with one nearly 4-year old, and weeks at home with no preschool are TOUGH. Not because I don't love my kid or enjoy time together or have things to do, but because it's exhausting being required to be on full volume from 5:45am till 7:30pm, THEN having to do all the cleaning etc. And because 3 year olds thrive on structure, a social group etc, and go crazy when they suddenly don't have that because it's the vacation. Yes, it's lovely if you have no work to do, can take them out twice a day and have an infinite supply of ideas for new activities every 20 minutes but it's exhausting to do that for weeks at a time.
OP - have you asked around what the other preschool parents do in the long holidays? I'm sure that many parents are also working and have similar issues with the holidays - is there any holiday cover? Can you arrange something privately with some of the same staff? I don't live in the UK but here there is optional childcare for big chunks of the preschool holidays.
I would be wary of splitting the week between two preschools. I think it's really important for kids to feel grounded and that the rules and routine are clear, and it might be difficult for him to be shuttled between different places. However, I would consider doing more days at either one if you can. Outside the weird SAHM world of Mumsnet, most kids this age are in full time preschool, and they absolutely thrive on it. Honestly, the more I read of self-righteous posts talking about how you should love having your 3 year old "help you out" around the home or drag around the supermarket with you, or feel you have to spend all day just getting out of the house... it's really hard to see how this setup is good for either parent or child. My 3-year old is in preschool 5.5 days a week. Last week they did an experiment to get dried beans to grow, they all baked bread, he built a "yacht" from huge blocks with his friends, they have a huge variety of crafts on offer, they learned about autumn, they played outside for hours and he seems to have spent half the week with his two new besties in the sandpit, and spent the whole walk home describing how to play the games that they made up together :) And... all that with no tantrums. I really don't think he's missing out because I didn't drag him to the supermarket, spend 30 mins in the car in order to get to an hour-long activity, or put him in front of the TV to take a break while I was cooking.
And guess what, he's still with me 5 hours a day at the very least before and after preschool, and during the evenings, weekends and holidays I have the energy for quality time with him: go to the library, bake cakes, go on a camping trip with him, do long hikes, travel, etc.