How old are your children and where are you planning to move to? Dexcom is a mix of self-funding and hospital funding. Ostensibly you should be considered for NHS funding as a child if you are hypo unaware or do competitive sport at a regional level and/or have had repeated severe hypo episodes (needing glucagon or ambulance). In reality, it depends on the hospital as many are very reluctant to fund. They are keen on the freestyle libre here, so you may be offered that funded by the NHS for your children. Some hospitals support the Tandem T:Slim, others don't and there are some hospitals which are more pump friendly than others. I would ask Dexcom about whether they can convert to mg/dl from mmol or vice versa, depending on what you want (assuming you all recognise mg/dl and want to stick with that?). All schools should have a policy for supporting children with medical conditions. In reality, this is also really variable. Lots of children in primary school have a teaching assistant who helps them with their T1D, but it can be difficult to get funding for this. School nurses are very rare and the school is expected to meet the medical needs of students from its teaching and admin staff, but as you can imagine this can cause lots of strife as it is often unclear who should provide the support and they need to volunteer to do it. Legally, the school needs to provide what support your child needs, but they cannot require any individual to take it on. It can be problematic, but in some schools it works well. I assume you are going for the state system? At secondary age, the child is normally expected to take everything on themselves with only emergency support (severe hypo) from the first aiders, but again, it is really variable. Diabetes UK (diabetes.org.uk not co.uk) has schools packs covering what should be in an individual care plan (although your hospital will probably have their own version) and also exam arrangements etc (but you will need to deal with your schools SENDCO/exams officer on this and will need to proactive).Your eldest should be allowed to access his/her iPhone to use as a CGM receiver even if phones are banned in the school as this is a reasonable adjustment under equality law, but you may have to fight your case, particularly as schools are less familiar with CGMs here than in the US.
If your hospital is in receipt of the best practice tariff (most are), you should receive four appointments with a consultant per year and an hba1c, BMI and blood pressure should be taken at each of these appointments. From age 12 they should have annual bloods and urine taken and will go separately for a retinopathy screening from this age. Some clinics do this from an earlier age. Psychological support should be available from a specialist psychologist and dietician and specialist nurse (DSN) support should also be available. The DSN will normally be your main contact with the hospital and you should be able to contact them for advice on insulin adjustment. Again, this can be very variable between hospitals, but the good news is, you can do your research and chose a hospital on the basis of how closely it actually adheres to best practice guidelines (and obviously practicality for you).
Also, if it is a permanent relocation, you should be aware that in the UK insurance companies only cover treatment of complications (e.g. foot abcesses) rather than the ongoing management of chronic conditions, so you will need to meet their needs through a combination of NHS funding and self-funding.
You should be eligible for DLA for them, and also for carers allowance if you or your partner is not working in order to meet their care needs. I'm not sure how you would go about applying for this on your return to the UK and there may be a lag before you can claim it.
Please feel free to PM me and I can recommend UK-CWD advocacy groups on Facebook. Three T1s to look after is a lot so I take my hat off to you.