It is legal to boost the mains supply to a property and the mains water pipes within a property in England, Scotland, Wales, and Northern Ireland, but only subject to very strict provisions. Fitting a single or double boost Shower Power Booster is an effective way to boost the mains pressure to your home and its legal.
If negative pressures are created within the service pipe feeding you property, this could result in contaminants and grey water entering the pipes. This is why the Bylaws set a strict a limit to be placed upon the volume drawn by the pump. The volume drawn by a pump is a simple way of limiting the suction of the pump (its ability to create pressures lower that those in groundwater surrounding the pipes outside).
Pumps which do not limit the volume drawn to below 12 litres a minute are illegal if fitted on the incoming main and would represent a significant risk to you health if fitted.
A single SP2B or double boost SP21S ShowerPowerBooster complies with the regulations and there is no permission required to fit is required a ShowerPowerBooster.
For those that want to read things for themselves this is what the regulations say:-
EXTRACT FROM WATER REGULATIONS GUIDE UK (including Water Bylaws 2000 Scotland)
Page 3.16 Regulation G6.2
Showers and other appliances which are supplied with water from a pump located either upstream or downstream of a mixing valve and capable of delivering more than 0.2 litres a second (12 litres a minute), may not be supplied with water direct from a supply pipe unless written approval has been obtained from the water supplier.
Page 3.16 Regulation R6.2
An installer will need to obtain the Water Supplier’s written permission before any pumped shower drawing more than 0.2 litres a second may be supplied directly or indirectly from a supply pipe.
In Southern Ireland the water regulations were the same until Independence however when the regulations in the UK changed after Irish Independence the regulations did not follow the UK. Despite this, pumps to boost mains pressure are openly on sale in Eire through major manufacturers so it appears that mains boosting is acceptable in Eire as well. I believe in 2015 or before it became legal to boost mains supplies in Eire.