Slacks is partly right about the flexible tails. Possibly a bit of confusion over the different types.
It's quite common for taps to include flexibles that are made to screw into the underside of the tap body. They have a threaded end with a small O-ring.
The other type of tails are made to connect to the supply (15mm fitting), while the other end has a standard (1/2 inch BSP) tap connector which fits onto a normal tap spigot.
So.......... it all depends on the type of tap you buy.
In most cases, plumbers manage (with great difficulty) to get at fittings on a bath mixer. It's a diabolical job, and I guess he has better things to do. Having said that; I can't judge from here, but maybe it's one of those cases where it really is built-in so much that bath removal is the only way.
Do what Sqad would suggest ... get a second opinion. :o)