It's not clear from your post if you are doing this for yourself or for children.
My general thoughts from experience with (now) older teenagers who have been through the whole process - be clear about exactly what the overall quote covers - not just the treatment and routine check ups but 1) all the extra fix/repair visits and 2) the post treatment bit.
We had numerous extra trips to fix broken bits - it just happens (sport/eating/general life) and for us these were covered. I know some friends under NHS or private treatment where there is a set number of fix visits per year/treatment. Practical issues too - can you then get there quickly and easily after school evening/weekend appointments to get this sorted. Lockdown availability too (fingers crossed hopefully not again).
Once treatment completed mine were left with a lifetime fixed bonded retainer behind the front teeth, very discreet but dental hygiene important, and shorter term plastic night-time retainers which has all been great for teeth stability - but still has required visits to fix and these have been extra charges.
We used Metamorphosis in Fulham, they were very very very good, definitely honest and highly skilled, and at the time equated cost wise with others, not totally local but easy enough to get to and on-street parking, not as easy on bus or tube from here, Balham/Tooting. Would definitely recommend them, but this was a few years back and they are now part of BUPA and that can make a difference to the very personal experience.