Positioning the ABV "as far away from the boiler as possible" is overrated IMHO. My ABV is only just past the (external) pump, so a total loop length from the boiler of under 2 metres or about a metre a side. Like you fitting it any further away would be infeasable as it would put it after the junction that splits the upstairs and downstairs feeds, so it would be either under the floor or in the ceiling rather than in the boiler closet!
Its primary purpose is to regulate the maximum differential pressure across the main header pipes supplying the radiator network despite changes in the number of radiators that are open, and to provide a minimum flow rate to the boiler and pump (so the pump isn't trying to pump into a dead end) even if all radiators are closed - which on an all HR92 Evohome system can happen under certain circumstances for a period of time, such as during system shutdown (heating off mode) where the HR92's will will fully close before the boiler relay turns off and (likely) before the pump overrun ends.
The ABV loop has a minor secondary function to dissipate a small amount of heat to help remove heat from the boiler heat exchanger under conditions where the radiators have all just closed and the boiler shut off but the pump overrun is still active.
However the amount of heat dissipated is very small unless the ABV loop length is very long and it shouldn't really be needed with todays modern low thermal mass heat exchangers. So whether you make your loop 2 metres long or 4 metres makes very little difference.
Also keep in mind that when you only need a small amount of heat to one or two radiators (say nighttime bedrooms) the longer your ABV loop is the more wasted heat there would be from the loop as the ABV would be flowing a lot in those conditions with little flow going beyond it to the radiators.
Personally I think being able to control all radiators in the house is a major benefit of Evohome, so I would be inclined to get an ABV fitted where convienient near your boiler and put HR92's on all radiators, which is what I did. Like you I still want to use the bathroom radiator in summer to dry and pre-warm towels and also prevent the bathroom fogging up, despite not wanting radiators on in any other room. With all radiators controlled I can do this, and I even have the bathroom radiator scheduled to stay on for a while after we leave for work to finish drying towels while other radiators in the house go off.
Before fitting an ABV make sure your system is not configured with an automatic pressure sensing variable speed pump - as an ABV does not necessarily play nice with a variable speed pump system as the pump will see the easy flow once the ABV opens as a sign that radiators have opened and that the pump speed should increase, which will cause the ABV to open even more and the flow to increase and the pump speed to increase further - which is exactly what you don't want.