a) On a flight say you depart Southend VFR for Exeter, ask Farnborough for basic service under VFR rules, if you later enter cloud (with an IMC rating) should you advise them from a legal and practical view point that you are no longer VFR and are now IMC and request a Traffic Service?
Above the transition altitude you need to be at the proper quadrantal rule level and you'd be well advised to tell the ATSU and get a radar service. I usually settle for traffic service as deconfliction gets you vectored all over the sky avoiding traffic unlikely to be a factor.
b) Enroute to Exeter, you are still in cloud and wish to transit say Solent zone, when you call them up although not on an IFR Flight plan do you state condition as IFR while requesting a zone transit?
If in IMC your only choice is IFR transit and the ATCO needs to know that as he must meet the appropriate separation criteria.
c) Should a transit be denied and you continue in IMC remaining outside of CAS you can if you so choose not talk to anyone?
Yes but that's a bit brave especially below transition level although above everybody should be at a quadrantal level and therefore vertically separated.
d)when you are then approaching Exeter and still in IMC would you advise again flight conditions as IFR even though not on an IFR Flight plan?
Yes and also agree what instrument approach you are going to use, Exeter do a nice line in vectored ILS and will probably offer it in my experience.