Whether or not it is legal in the UK will, I think, depends on the weight of the drone and you may be limited to an under 250g drone.
As for height, all legal height limits in the UK in a 'flat area' are measured from the ground beneath the drone { AGL}, there is no ( building height +AGL allowance). In a 'hilly town' within 400ft of the closest ground might apply.
Besides, I think that the ( building height +AGL allowance) in the USA applies to their "part 107" qualified pilots and applies only within 400ft horizontal feet of a structure. Beyond that distance I believe it reverts to AGL only.
In addition it would be advisable to get the building / land owner's permission.
I would be wary of taking off from a building because of steel in the structure affecting the drone compass etc. or, at the very least, check that the artificial-horizon/attitude-indicator/radar indicates that the non flying drone is pointing in the correct direction.
I would also suggest third party insurance would be wise, personally the "what if something went wrong" considerations mean I have not and would not attempt such a flight.
As for ground level launching/catching, I would be wary of attracting the attention of revellers/drunks.