Honeywell to Nest thermostat (wired) UK

[Phantoms]

I am looking at the compatibility of the nest thermostat to replace my current Honeywell T3 wired thermostat. The thermostat runs a combi boiler

It’s a 2 wire only system, however there does appear to be another brown wire in the wall box connected only 1 side to a connect block.

 

[trojanhawrs]

It's different, that cable will run direct to your boiler. The nest come with a heat link you can install next to the boiler and you can repurpose that cable to go from Nest - heat link. The heat link is wired to the boiler.

 

[RandomGrinch]

The 3rd generation Nest will be compatible with your combi, but there are different ways you can connect it up - some ways are easier than others!
It depends on what you wish to achieve.

As above, a common way to mount the Heatlink is next to he boiler - the existing wiring to the Honeywell, can then be utilised to power the Nest thermostat (through connections T1 and T2), if the Honeywell was placed in a convenient location.

The Heatlink could possibly be placed where the Honeywell is, but it would need a neutral connection from the existing cable - there may be a grey wire hiding behind the Honeywell that could be used for that purpose.
However, you would have to ensure that the boiler is mains switching and not (extra) low voltage switching, first.

Alternatively, and very simply, the Honeywell could be swapped for the battery operated Heatlink of the Nest Thermostat E.