So the boiler has a double fault situation that can't be covered by bs7671, no earth and an l-e fault, therefore it's a very dange situation.The boiler in the basement develops
A L-E fault, (lets say it has no cpc either) this is not RCD protected the pipework is copper apart from the incomming pipework..
The towel rail and bathroom location is irrelevant, basically this is the"all conductive surfaces are dangerous" argument which is true, and applies regardless of whether they are bonded or not. Argument has been made for bonding conductive things that are built into the fabric of the building, but you have to draw the line somewhere otherwise you're bonding teaspoons before you know it in case someone pokes them in a toaster.The pipework in the bathroom is now live and the customer would get a shock if they touched the new towel rail and taps at the same time.
My personal opinion is given the choice you should avoid conductive surfaces especially ones that pop up around the house eg pipework and metal frameframe partitions.