Just to follow up on this thread, we have been using Three as our broadband connection since the 27 January. I've had no complaints from my daughter, and I really haven't noticed any difference in use myself, apart from the IP address changing frequently.
The IP address has changed multiple times, I even forced some changes, but only once got an IP address that was externally accessible, and this was using the 3Internet APN. Not a major problem, but I prefer to have access to my network when away from home.
So today I made that phone call to VM, and was eventually offered M350 for £38.50 a month (a rise of £1.50), on an 18 month contract. Not particularly keen on an 18 month contract, but given the time of year it will be about 11 months until the next price increase get out option comes along, by then we may have FTTP or 5G available.
So I've accepted the above offer, yes I could have saved some money over the year by moving to Three (£10.60 a month after cashback), but in the grand scheme of things it wouldn't be much, and as others have mentioned our 4G could get a lot worse, or we could have 5G become available, but I've decided to play it safe, and it keeps the network tidier as well.
I do have an eBay special 100GB EE SIM to experiment with - we have line of sight to their tower as well, and it's closer than the Three tower, once I've tested that I'll post the results.