I don't believe the alternators for UK bikes were any different than the US/NA models. Being able to switch off the headlight will certainly help with battery recharging when lighting is off.
I don't know what standard bulb wattage is for your bike since you didn't tell us what you have or what model/year bike you have.
You should measure what voltage is getting to your headlight compared to what is at the battery. Aged/worn connectors, switches and fuse terminals can all lose some voltage. And, if the headlight doesn't get full battery voltage it will be weaker than is should be, and "upgrading" to a higher wattage headlight will only create more voltage loss through those same connectors, switches and fuse terminals.
FYI: the watt ratings on the bulbs are when 12.8 V are applied to them. More voltage = brighter, less voltage = dimmer.
I'm pretty sure that electricity adheres to the same electrical and physical laws we have in the US.

Cheers,