No HW in loft - Low Water Pressure?

Valiant VCW242E Combi. HW and CH fine except sometimes no HW in loft and noticeable that water flow is not great (but HW always OK ground & 1st floor). House is NE London and quite high up. Thames Water said if can fill 1 gallon container in 30sec (1st outlet, street level) then pressure OK. Done test filling 1 gallon container, downstairs 20sec (cold), 30 sec (hot), but in loft 1 min (cold), 1min20sec (hot). At time of test unable to get hot water in loft, but CAN get HW in loft by leaving loft tap open and running HW downstairs and then shut downstairs tap - suggests that flow required (by boiler) to start delivering HW is greater than flow to keep it going.
Question - are there any settings on the boiler I can adjust to maximise HW flow - so HW pressure is closer to the CW pressure?

Water supply in areas such as London do suffer at peak demand so with the height u require if there r times u cannot get any flow through the hot tap what is the cold like at thet time. The boiler will require a reasonable flow to start.Is your incoming stopvalve open full in the house & the road as this will impead flow during peak demand. How olds the boiler incase the inlet filters have a restiction on them.If theres been work on the watermains in the street or even installation (flux or copper/plastic )filings on it it will all add to the problem.May be non of the above but as a fitter i would have to rule them out.