They don't work too well under the bath, thats why you won't see too much about them being used under the bath. My old house had one (B&Q jobby by previous owner), waste of time as made a lot of noise for bugger all increase in pressure, probably being due to the long run of 22m pipe from the tank not supplying enough flow. Was connected to the 22mm pipes supplying the bath then 15mm pipe to shower head, the shower head had been changed to one with not too many holes, probably to keep the pressure up as when I changed it to what I thought was better, more holes, the pressure was less (flow more) and the pump tended to cavitate, implying not enough flow in, unless you turned the shower valve down a bit. Also affected if someone flushed the loo or ran the bath room tap.
Anyway replaced it with a proper pump in airing cupboard supplied direct from hot and cold tanks via 22mm pipe (use plastic Hep2O push fit, very easy) and back up into loft in 15mm to shower in bath room. A monster skin ripping floor flooding shower if everything was turned up full !!!! I used a
12V Watermill pump