I'm not a plumber but..
If running a cold tap drops the flow rate on the hot then that sounds like a problem with the incoming main or the pipework between main and boiler. Check all valves are fully open.
It sounds like you already suspect the incoming main is the problem... Have you measured the static pressure and flow rate where it enters the house?
1) If the
static pressure is good but the
flow rate is poor then two solutions exist. In order of preference.. a) New fatter pipe from main to house. or b) a pressure accumulator..
http://www.plumbingpark.co.uk/plumbing_hvac_article12316.htmlhttp://www.modbs.co.uk/news/fullstory.php/aid/5100/Are_you_feeling_the_pressure_...Note that I'm not talking about a thermal store. Sometimes those are also call accumulators.
2) If the main in the street is the problem and
both the static pressure and flow rate are low then it might be possible to use an accumulator with a pump but I've not investigated those.
The problem will be finding someone locally who knows how to test the mains properly and correctly specify/size an accumulator. I didn't need one but the plumbers who built my house had never even heard of them. They are similar to the expansion vessel on a central heating system only bigger.