I've been using the dexcom G6 for about 3 years , and generally find it very reliable, though I occasionally have issues with new sensors under-reading as they settle in, and though dexcom does allow calibration (very very useful) you can't do it for the first 24 hours. (I was a libre one user before that but had to stop using it due to an allergy to the sensors which also made them completely inaccurate for my body).
I know I read a thread by someone trialling the dexcom one and decided that I needed the features that they've cut from it. (In particular the follow app, which lets me send my dexcom data to my watch face).
The sensors do fail sometimes. Recently twice, with readings that never got higher than low, and I suspect something to do with my body and the placement of the sensor. I have switched from my stomach to my arms, because I think that maybe I was overusing my stomach (combo of a bit of hyperlipotrophy and a bit of weight loss). But given that I had to stop using the libre because of a reaction, I think maybe my body is just fussy with cgms?
This is not necessarily a problem for anyone else.
Dexcom customer support in New Zealand and Australia are a 24/7 helpline staffed by a team of dexcom users, not sure how it works in UK.
Note that the G6 only sends a signal out every 5 minutes, so if you've been away from your phone and want to pick up missed data you have to wait till the transmitter does its next signal, unlike the libre where I believe you can scan at any time. (Not sure about the G7).
I've been very happy with the dexcom.