I'm not a plumber but..

I think if I were installing a new cylinder I'd be tempted to get one with extra coils to make adding solar hot water easier. If the new government get their act together with RHI they may even pay you cash for every KWH of solar hot water you collect and use....

http://www.rhincentive.co.uk/

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Yes it could well limit the flow rate you get.  Your existing system probably has at least one tap directly on the incoming cold main, typically the kitchen tap. Measure the flow rate you get out of that in L/min and it will give you an idea of what you will get from the new system in total.

If the flow rate is poor you can do one of three things...

a) Get the incoming pipe upgraded.
b) If the static pressure is good you can install a cold water accumulator/expansion vessel...

http://www.tradeplumbing.co.uk/Cold-Water-Accumulator-Improving-Water-Pressure_c...

They "slowly" fill/charge up off the mains via the narrow 15mm pipe and then sit there in your loft ready to boost the flow rate when you turn the tap on. However they only provide a temporary boost to the flow rate. How temporary depends on the size of the accumulator and other factors. Big ones are expensive and upgrading the main is a better choice if possible.

c) Stay with a stored cold water system and add a pump. Either a whole house pump or one specifically for the bath/shower(s). Unless things have changed recently you can't pump the mains directly.

With the existing 15mm copper feed, we barely get 11L/Min at the moment, so it looks like the pipe upgrade is next (I'm re-laying the driveway anyway & that's where the pipe runs). The pipe in the street was upgraded to new, plastic stuff last year, so presumably I only have to do the bit from the stopcock to the house.
I have a fully-pumped system at the moment (salamander) but I want rid of it.
The existing boiler is in the Bathroom (can't do that now?) so replacing it and moving it to the loft space is the ideal opportunity to sort the whole lot out.
If (when) I replace the 15mm copper with a new plastic feed, can I put a stopcock in a wall-mounted enclosure on the outside, like an electricity or gas enclosure?
Any recommendations on boilers?

Thanks for the info so far Scruff & CW - I'd like to ask a couple more questions please?
1) The bungalow has solid concrete floors, so apart from major floor excavations, any tips on how to get the pipe in?
2) The static pressure is very good I think (it will wreck a cheap hosepipe anyway) To determine what the static pressure actually is, do I ask the water authority or can it be measured with a gauge of some sort?
- any recommendations on choice of boiler? - or recommendations of what not to buy?
Thanks very much
Wozzer

Thanks for the info Scruff - Excellent.
- I know what is required now, and will have a chat with the plumber chappie down the road to see if he will come measure the water pressure.

Thanks again for the help.

Regards,
Wozzer