I just had the supply line from the street to the house replaced as it was leaking badly. The plumber said we needed a pressure reducing valve but didn't fit one. I was wary of turning the water up too far until we can get a valve so the pressure is a little low right now.
The problem I'm having is that the water in the upstairs shower is now only lukewarm. The faucets downstairs are all OK it's just the shower upstairs that's the problem.
Could this be caused by the supply pressure to the house being low or is there something else wrong?

I was wary of turning the water up too far until we can get a valve so the pressure is a little low right now.
Oh no it's not! By closing down the house shut off all you've done is reduce the volume coming into your home. Then pressure remains high until the system begins to run out of volume which you have reduced at the house stop. High water pressure can put a strain on elbos and faucets. Have the PRV valve (see image)installed ASAP.

The faucets downstairs are all OK it's just the shower upstairs that's the problem.
You have localized your problem. Shut the water off to the shower valve and open it up. Remove the cartridge and check the inlet ports for trash. Now, while the valve's open turn the water back on to flush out the supplies and valve body. Reassemble and test. Better now? If not then click back and tell me about it. Good luck, Tom

Thanks for your suggestion, Tom.
I have a PRV coming in a few days and will fit it next weekend. Meanwhile I'm using the main faucet to regulate the flow. I just kept opening the valve until it the faucets were running at the flow they were before the supply line was changed.
I opened up the upstairs shower valve but couldn't find any dirt or debris inside.
I put it back together and it's the same. Also I have just noticed that the downstairs bathroom faucet is running lukewarm, too. The kitchen faucet is hot and the boiler is running OK.
What should I do next?

":Meanwhile I'm using the main faucet to regulate the flow." this does not regulate anything but volume. What exactly is your psi according to the plumber? A new supply line should not change your pressure.

The plumber said that he put a hose bib on the supply line and the pressure was 125PSI. He said it should be 65-75PSI.

Most of us would shoot more for about 45 psi. Wonder what your neighbors psi is since he is on the same main, do oyur neighbors also have a PRV?

I don't know about the neighbor. I'll ask him when I see him.
The PVR comes set at 50PSI so when I fit it, I should just leave it as it is, right?

Any more thoughts on the upstairs shower hot water running lukewarm?

Let me explain about the average house PSI. All city water towers are 100 feet high. There's a reason for this. At 100 feet high the head pressure at the base is 43.4 PSI. Add the depth of the service line and you get 45 PSI. And now you know why water towers are 100 feet high and why the average house pressure's 45 PSI. However, Acceptable limits are between 40 and 80 PSI. Good luck, Tom

If the average house pressure is 45PSI how come mine is at 125PSI? Does that sound right or was the plumber trying to generate a little more work?

":Meanwhile I'm using the main faucet to regulate the flow." this does not regulate anything but volume."

Does this mean that I could still burst pipes or fittings with the faucet only half open? The PRV won't be here until Friday. Should I close the main valve to stop potential disasters?

I don't want to cast a shadow on someone else's work but I do find the 125 psi a bit out of line. You can do your own test for about $15 by buying a gauge and bib to attach to your garden spigot. If you want to try we can help once you have your gauge.

It's not very often I disagree with Bob But his comment about house PSI of 125 PSI is way out of line.
I say a top limit should be 80PSI and here's what the internet says,

If the static pressure is more than 70 PSI I strongly suggest you tap the water supply after the regulator. Pressures over 70 PSI can damage some equipment commonly used in residential settings. If the pressure is over 70 PSI and you do not have a pressure regulator your house has too much pressure.
Sorry Bob, Tom

Tom, not sure I understand your point. Are you saying 125 psi is not out of line? "I do find the 125 psi a bit out of line" I agree 80 is tops with 45 psi being what I shoot for.

I responded to this statement in one of your earlier posts.
I do find the 125 psi a bit out of line. My bad! I misread your statement. You have my sincere apologies. Hugs? Tom

No problema señor, I said a bit out of line instead of way out of line so as to not overly alarm the OP. I really doubt his psi is 125, that's why I suggested doing his own test.

I called the water company and they told me that the pressure in that area is at 122PSI so I think I can assume that the plumber was right when he said 125PSI.
There was no regulator there before he did the work. I have one coming this week and will fit it at the weekend.
Meanwhile, should I leave the main faucet about half way open or should I be closing the valve completely when I'm at work and not in the house? The flow rate in the house is lower than it was before the work was carried out.

And I still have only lukewarm water in the upstairs bathroom.

My PRV has arrived and I will fit it at the weekend.

Meanwhile, I have set the thermostat on the boiler to max and when I turn the upstairs shower on to max hot it is still not very hot.
It's hot enough for a shower but should be far hotter.

Will fitting the PRV make any difference to the water temp?