Hi, I can answer your question, lets handle them one at a time. Click 'accept' when you are satisfied.. or we can get another expert.
Hot water pressure is lower than cold water in UK systems for many reasons. including higher corrosion and rusting rates in the hot water lines. There *are mixing valves that can be installed (with repiping required) to achieve many of the goals you have laid out... you generally need a pump with those as well. Investment will be in the 1,500 to 3,000 pound range.
Meantime can you tell me how old your plumbing system is and if its copper, galvanized or a mix.
I will direct this to our UK expert Rick, 30 years in the UK plumbing business.. and stay around to be sure he gets it.
Do not click the accept button.. wait for Rick or another UK qualified plumber.
Hello again, after 9 years, any copper to galvanized joint without an electrolyic union and even some with such electrically isolated unions will have seriou corrosion and rust build up inside the galvanized pipe.. that along with scale build ups can seriously hamper water delivery.
The first place to look is at the water heater fittings...if those have galvanized nipples that will be a primary choke point, replace those... and look the hardware store over for their range electrolytic connectors, stainless flex hoses for connnecting water heaters with a plastic flanged gasket to isolate electrically or a hard brass and steel union.
Beyond that is simply scale build up in the old galvanzied pipe... at 60 years unless you are blessed with very soft water in the area, those will be 90% full of scale, and rusting through at the threads.. prone to springing leaks. A move to all copper piping will be your best bet.
That should be fixed first. Beyond that as you indicated an interest are various pumped and mixing valve strategies to keep hot water circulating in a loop so that when you want hot water at a tap...its already there. .. no wasted hot water while a 100' long pipe warms up before you get hot water... with that approach you also need good hot water pipe insulation. Your house lay out, skill set, and budget will define what you go with there.
Regarding the device.. I can list half a dozen. but to fit you system is another story.. any competent plumber can look the job over and fit you up. Penn Controls makes one, its a 3 way temperature controlled valve, applied at the farthest fixture it will improve performance of all the rest but needs a pump in the system to force hot water to the cold water side.. ..there are others. the application though most be seen to specify anthing proper.
The rest is an engineering project beyond the scope of just an answer. but a good plumber on site can do it easily.