What type of shower?

I want to install a shower on my upstair floor but I have a few problems in deciding the best type of shower.

1) The cold water is mains pressure throughout, which is, I believe, not suitable for a power shower, nor a normal thermostatic mixer shower.

2) In any case the upstair floor is very low sealing and this means that there is not enough head (less than 1m) for a simple mixer shower.

3) I do not fancy wiring a electric shower as it is pretty far from the CU and being an old house there is no obvious route.

So my question is:
Can I just take a spur off the cold water tank which serves the hot water system and use this as the cold supply for a power shower. If that's OK, what is minimum size of tanks (both hot and cold) I should have to supply a power shower (say a 1.5 bar one)

Any other suggestions?

So my question is:
Can I just take a spur off the cold water tank which serves the hot water system and use this as the cold supply for a power shower..
answer is No. A power shower needs to draw its supply, both hot and cold, from tanks so that it is not subject to problems with mains pressure variations.
If you have a hot and cold water tank fitted in your home whether it's in your airing cupboard or in your loft, weather it's heated from your central heating boiler, or heated with your immersion heater. Then the answer is usually yes. There are limitations however on having a power shower fitted ! The first one is your hot water tank the average size is 36" high and 18" across and holds about 50 gallons of hot water. The second one is your cold water tank it must hold a minimum of about 25 gallons of cold water, not because you will use this amount of water with a power shower, but because your cold water tank is feeding your hot water tank and your shower under gravity at the same time, but once the hot and cold water supplies hits the pump it is fed under pressure to the mixing valve to stables the temperature and then out of the shower head. With the average power shower the flow rate would be approximately two gallons per minute, that's over six times higher then the average electric shower, so you should be able to get two good showers one after another out of the average tank full of hot water.

Plumbing requirements Your new power shower has to be connected to the hot and cold water tanks, and both the hot and cold water supplies to your power shower should be of equal pressure. Ideally a power shower should be fitted as close as possible to the hot water tank, but if this not possible then try to keep the hot supply pipe as short as possible, avoiding high points in both the hot and cold pipe runs because this can cause airlocks in the pipe work supplying the power shower with water, airlocks will reduce the performance and flow rate of the power shower and could damage the pump. As to the cold water tank ideally the cold water tank should be a minimum of 3" yes that only three inches above the shower mixing valve this is called head height, with some of the all in one power showers, Although if this is not possible or the shower is going to be fitted above the hot and cold water tanks say in a loft conversion , then this is called a negative head installation, and will require specialists equipment to be fitted.

So if you are in any doubt get it installed professionally.

Sorry, I probably did not use the right technical words but I think that you are saying yes to my question.
Currently, the cold water tank is only used to feed the hot water tank. All cold water taps in the house are connected to the mains. So for a power shower, I want to take the cold water from the cold water tank instead of the mains.
For me to do this, can I take a spur from the pipe (using a tee) that supplies the hot water tank or should I connect it directly to the col water tank itself?
Also, the hot water tank is downstairs and I am fitting the shower upstairs, so there is fair distance between the two. Can I still go ahead?
From your answer, the size of the tanks and the head height are all fine.
Thanks for your help.

You will need to take the feed from the tank ensuring the hot feed is higher than the cold. This is so that if the float valve fails your hot supply will run out first and you will not get scalded. Arrange your pipework accordingly.
The pump will accomadate your cylinder downstairs.
Cracking answer from Will

Thank you both for your great answers!
I just need to get working!