Lots of people have done this. The new controller [e.g. Hive] would replace the existing "timer", and the existing room thermostat would be removed. However, removing the room thermostat would leave the wiring 'open circuit' and the heating would not operate. So, a link needs to be installed to bridge the wiring where the original thermostats 'live' and 'switched live' wires have been removed from. This completes the circuit and allows the new system to take full control of the time and temperature functions.
If you need any further assistance, once you have decided on which new thermostat you would like to go with, post back and include photos of the existing "timer", "room thermostat", and the wiring connected to both of their terminals, then someone will be able to provide you with further help.
The boiler you have isn't really on issue, as it's not being changed. It's the items that you are replacing / removing that are important as the new controls simply need to provide the same electrical control functions that are already there.
This enquiry would have been better placed in the plumbing and heating section as that's where the guys in the know about these things usually hang out, there's a good possibility they might not see it in this 'General Discussion' section. So I suggest if you do want further help, start a new thread in the plumbing and heating section with the necessary information /photos etc.,