Step one have a look as the boiler installation instructions and see if it has option for OpenTherm, if it has better wiring to use it, if not as @MeldrewsMate
says not much room inside Nest Gen 3, I put a wiring centre below it so one terminal one wire. Likely it will replace the LP711 and the SCR with wire into LP711 so can go completely.
But you have three options really, unlike the LP711 you don't need to set Nest at the Heat Link, so it can be mounted where not easy to read and alter as there is nothing to read and alter, so could go right by the boiler or in airing cupboard, so option one is start from scratch and mount near what it controls.
Second option is mount where LP711 is and used a USB cradle for the thermostat bit.
Third option is hard wire the thermostat bit, I did the latter, still no sure I did right thing, but boiler was on another floor and the pair of wires was already installed.
From what I remember of the LP711 it can be configured as 16 option or 10 option with a switch and/or slider, so can be used with C Plan or Y Plan or S Plan, Nest Gen 3 can be used with C Plan that is what I have, but there is no circuit diagram on how to wire as C Plan in the little book that comes with it, so before starting you need to work out what you have.
C Plan often no motorised valves or tank thermostats on the DHW.
Y Plan a three port motorised valve.
S Plan 2 two port motorised valves.
Combi boiler no control of DHW by Nest the boiler does it.
All central heating is a compromise, but some thermostats are better suited for some systems, There are three really good ones, the Honeywell EvoHome, Drayton Wiser, and Tado the latter I know very little as to how it works, but it seems the Drayton has the best programmable TRV head which works out how the rooms heats and remembers it, so is the fastest to heat room without over shooting. But non are designed to work with zone valves.
EPH however can work with OpenTherm and zone valves with a master slave configuration, but it does no it seems link to TRV heads.
Nest is an odd one, it has got OpenTherm but will not with with either zone valves or TRV heads, it is a stand alone only, a very good stand alone, but does not work with TRV heads.
Hive works with TRV heads but does not have OpenTherm, it does however have a configuration to work with old C Plan.
Worcester Bosch is like the Apple Mac of the boiler world, a good boiler but with not work with common third party thermostats, having no OpenTherm but does have it's own e-bus system, but Hive system works well with Worcester Bosch allowing the return water temperature to control the boiler.
But the house also matters, my old house open plan Nest was great, mounted by the arch between rooms worked A1. This house really do need TRV heads which link to wall thermostat, there is no room on centre level without a door to the outside, and only the hall and dinning room have no alternative heating, I made a mistake fitting Nest Gen 3, I was told it worked with the Energenie MiHome TRV heads, and in a way it does, but not in a way which can be used.
Non of the systems are cheap, as to if worth have each room with independent control is worth the cost I don't know, I have 9 programmable TRV heads, and they do stop rooms heating up when not required, but they can't heat a room if boiler not running, so in my case the hall is room with Nest in and it and the TRV plus lock shield setting in the hall controls whole house, when I moved in central heating was a mess, so never used central heating without Nest, however my hall is too slow to cool, so key is to ensure it does not over shoot.
Nest with all its cleaver algorithms can't work out if hall doors are open or closed, so it can never learn how to control as it does not know how many of the 5 doors into hall are open or closed. And with an open fire and a pair of double doors to outside in living room no good putting thermostat there. Dinning room also sliding doors to outside, and kitchen door to outside and a cooker. So this house really needed linked TRV heads.
Also live style, I am retired and have not left the house since March, other than medical appointments. If my wife and I were going out to work, then the Nest geofencing would be really good, however since Nest also has a built in PIR it needs to be in an area where it will detect movement, so hall is ideal as all rooms lead into the hall. This may not however work out with all homes, in my sons old house where the thermostat was mounted it did not register there were people in the house.