To summarise:
There is nothing in UK law to prevent an aircraft being flown from the right seat, provided that all necessary controls can be reached, unless there is a specific prohibition in the POH/Flight Manual.
The PA28 can be flown perfectly safely from the right seat and all controls can be easily reached (including the fuel selector). I have, on more than one occasion, flown a PA28 solo from the right seat with no difficulty.
There is nothing in UK law to prevent a passenger handling the controls of an aircraft on a private flight. The reference to JAR-FCL 1.010 quoted by Ryan5252, apart from not being UK law, is irrelevant as simply handling the controls does not automatically make one a 'flight crew member'.
The only considerations are the insurance requirements and the advisability of familiarisation training in the right seat.