You can tee off the mains, but you have to comply with the regs, try the Watersafe website for approved plumbers. http://watersafe.org.uk]
Just put your postcode in and loads should come up, but plumbers can be a bit precious about digging, so to save you some pennies I would dig it out yourself.
Probably lead pipe, maybe copper I would say, so might be a bit of a ball ache Teeing into the lead for a non professional .
Check what pipe you have under your main internal stop tap to see what it is.
Victorians used to run the mains up the side of the house in the same trench as the drains...
Have been listening to 1001 albums and into the 60’s at the moment and uncovered this gem. Never heard of him before, but really good sound for the time.
Took a woman out once for a drink or few, she turned up in a lovely short dress, lovely long legs.
Later on that evening, when the drink had suitably circulated we were sitting side by side I noticed a peanut on her thigh , which I rather gallantly flicked off.
Said peanut, just wobbled and...