gaf20,
Whether or not you choose to make a PAN call or not ATC most definitely will want to know if you have any sort of hydraulic issue (even if the a/c handling is not affected) as the runway must be inspected following your landing.
Taking LHR as an example... if you landed normally mentioning nothing until you vacated when, "oh by the way we did have a hydraulic issue", there'd likely then be multiple go-arounds behind you and a good few minutes of chaos.
However mention in good time that you have/had a hydraulic issue but require no special handling then expect a normal approach but with a suitable gap behind you to allow the runway to be inspected after you land and no chaos.
There's nothing in the Terminal Control MATS Pt 2 that says you must declare an emergency but it may be the case that the airport Pt 2 states that any hydraulic issue is to be classed, as far as they're concerned, as a Full Emergency. That doesn't really affect you but rather just gets the appropriate services on the ground ready just in case. Possibly overkill but it's good practice for all concerned.
ATC will not encourage you to divert anywhere but ATC can be requested to pass a message on behalf of the BAA (at Heathrow or Gatwick) to request that an a/c with technical difficulties that may result in a runway being blocked consider diverting somewhere else.
That message will not be passed though if the a/c has declared an emergency, has reported a fuel shortage or the weather conditions elsewhere precludes a diversion.