I’ve just done some quick DIY (5 minutes) in the bathroom and fitted an eaga ShowerSmart, and the good news is they are free of charge to UK householders. I applied for mine so long ago, I forgot about it!
It’s a small metal unit, which restricts the flow of water through your shower head, saving water and energy.
Read and apply for one here at http://www.shower-smart.co.uk/
The eaga ShowerSmart is suitable for use with non-electric mixer showers or bath/shower mixer taps, provided they run off the mains water pressure.
It’s supplied with two washers, good instructions, and a small plastic bag which you use to check if your shower is in-efficient.