Pumping the Mains

I know that this mustn't be done.
But why not?
I've got an electric shower in a loft conversion. The mains feed is so low pressure that most of the time the 0.5 bar low-pressure switch on the shower cuts out.
What would be the problem in fitting one of those low-presure electric showers that has its own built-in pump, like the Mira Elite?
(I realise that installing a separate tank and pumping from that would be best, but there is no room for even the smallest tank.)

It is against the water regulations as you need permission if it pumps 12 litres per minute plus.
The reason being you cannot pull water at a higher rate as the flow can not make it up and you would be pulling air and may cause a vacum which may lead to backflow containimation.
Bascially if the water flow is not there in the first place you cannot magically increase it with a pump.

You will have to tank it and then pump from there making sure it is big enough no to run out as it is being used.
P

another good reason is that when you would be having a shower next door wouldnt be able to get any water

Thank for your responses.

I do see the problem of possibly reducing the pressure below atmospheric and drawing in contamination.
However, as I (no plumbing expert but from fire service experience knowing a fair bit about water flow calculations) see it, firstly, the flow to this shower would be a negligible amount so I can't see it interfering with flow to any other property. Secondly, a small shower pump would only be replacing the losses due to friction etc in reaching the top of my house (The water does flow out of a tap in the same room at a reasonable rate, it is just not at enough pressure (0.5 bars) to keep the shower unit on.)
What is wrong with this thinking? Have I forgotten something, will there be a disastrous effect like my hot water cylinder imploding?

Thanks again in advance for any help!

If you have a HW cylinder then presumably a CW cistern, so why not pump that? Electric showers only draw about 4 litres/min.

If your pressure is really that low call your water provider to discuss it. They'll tell you what the pressure should be in your area and what your pump options may be.

Thanks, Chris.

Yes, I do have a cold cistern but it is below the height of the shower and so would need a negative head set-up.
Also, I wouldn't relish the thought of lifting the chipboard sheet floor in the loft to connect to the two (about 8m apart)
I just thought that since, as you say, an electric shower has a low output, helping the flow a bit with a pump might be a convenient solution. The Mira Elite info states that it shouldn't be connected to mains pressure but in my loft the pressure is negligible.