Check there isn't a pump already, there would normally be a pull switch in the bathroom if so. Look in bottom of airing cupboard for a pump.

If there isn't a pump, first try and see why shower sin't working properly. Next step is descale shower head or just buy a cheap one, they all have the same screw thread.

Only if those investigations fail consider another shower.

You can buy a thermostatic mixer shower which has an integral pump. It would require an electrician to be involved in fiting such a shower with the new Part P of the building regs now in force. One make is New Team 1,000, there are others.

12 ltr/min is very good, but just because that is what the shower will give doesn't mean it will in your house if you don't have a good flow now you aren't likely to get one without a pump.

Paul