Anton Sprint V3, same as the V2 but with Bluetooth.
British Gas use 8000 V3's and about 2000 TPI instruments.
We see approximately 200 Sprints a week, very few problems with them. Easy to use, firmware updateable to add addition functions to the soft control buttons, something you can not do on mechanical switches. Expandable to add NOx if and when required, USB port to connect additional probes, so far the gas leak probe but they are working on other. The USB port is used to down load the stored logs. With a few authorised service companies, choice of who to send to for service and recalibration.
Another make to look at is Testo; either the 327 or the 330LL range. Both have menu driven function keys, once set up, easy to use. At least one authorised service company in the UK. They may be more next year.
The 330LL version has long life cells which have an expected life of 5 to 6 years. This keeps your calibration cost down, but the instrument is a lot more expensive than the Kane or Anton instruments.
The Kane 455, direct CO2 and 5 year life on CO cell. Not sure on the benefits of direct CO2 and calculated O2 readings having read about excess air and how it is calculated. Reasonably high service costs, but no cells required for 5 years. Kane do not have authorised service companies, so be careful who you send it to for calibration.
TPI instruments. ….nice colour. Apart from that I know too little about the instrument to comment. Some customers happy with instrument.
Do not go for some obscure manufacture, they may not be any UK based support. Stay with the main four.