If you have a split load RCD board (2 rcds) then best practice would be to have upstairs lights and downstairs sockets on i.e. rcd1 and then downstairs lights and upstairs sockets on rcd2.
The reason for this is that in an event of a fault/nuisance trip detected by the rcd, you will still have live sockets within the area to plug lamps into.

It is not a legal requirement, some properties are split upstairs and downstairs, some are split front and back of property, It would be difficult to change an existing system and also unnecessary. If you were wiring a new property or having a complete rewire you may wish to run front and back, there are advantages and disadvantages to both systems. If you are concerned there are fire alarms which are mains powered and have an emergency light in case of power failure, installing one of these may provide a more cost effective solution. It would be wired into the existing lighting circuit. I hope this may be of some help.

My suggestion (which depends on how good or bad your consumer unit is) is to have an RCBO for the lights downstairs and a seperate RCBO for the central heating.
An RCBO (correctly installed) should trip when a fault occurs on that particular circuit ONLY, this helps prevent problem trips.

I agree with ABC Electrical, RCBO (Residual Current Operated Circuit Breaker) for Downstairs and an RCBO for Upstairs is the best way 2 separate lighting circuit.
protection against overload and short circuit.