Service varies a bit by region, the likelihood of true 24hr turnaround on service callouts is going to depend on how close you are to the service centre, being that most machine tool vendors have only one service centre in the UK, and have a service radius that they will travel within, and hire third party contractors outside of that radius. Whereabouts in the UK are you?
Another thing to consider is that if you are a small shop, the likelihood of receiving true 24hr turnaround is pretty slim, as all their bigger customers will get priority. That's just how it is unfortunately. Bearing that in mind, you need to go with one that has a good reputation for taking care of small customers. I'm not truly aware of that landscape in England, but I can give you a list of who to avoid on that count, it is a short list, and it is basically "Mazak".
How has your experience been with XYZ? They are strongly represented, and their Siemens based machines should be pretty solid. I have never used shopmill, but others seem pleased with it. Based in Devon IIRC, but I think they may be one of the few with additional service centres and/or showrooms.
Hurco have never left us hanging, but being pretty distant from their HQ we never received (or expected) 24hr turnaround. Service was always scheduled with a more local third party, apart from twice when they flew one of their own engineers up.
Doosan are well represented by Mills, and are good solid machines, but the basic VMC's are all Fanuc equipped and Fanuc's conversational is pretty pathetic. Fanuc only if you will embrace CAM IMHO.
Member Barbter should have a pretty good handle on service support in England, but he doesn't frequent the site too often any more.
Hurco VMX would likely be a good fit for you because of the strengths of the control, but you might need to temper your expectations of a true 24hr turnaround, and honestly, unless you are Rolls Royce or GE or some such, that's likely to be the case across the board.
Okuma and DMG Mori both have UK showrooms and service centres, but suspect those are not on your shortlist. Different price bracket, and neither have outstanding conversational capability.
Most of the Taiwanese and Korean machines are represented by somebody or other in the UK, but again, those are pretty much universally Fanuc, so can be generally ruled out by your criteria.
I have no idea how Haas support is in the UK, but they would pretty much always be right at the bottom of my shopping list anyway so I've never enquired.