Can't advise on a particular local company.
Just to say that most companies of plastic conservatories sources their parts from one of the big frame companies such as Ultraframe plc. The raw materials therefore don't vary too much.
What differential price you are therefore paying for is the quality and competence of the company doing it and the quality of the bits within it. I.e. look carefully at the quality of the base they propose to construct, don't forget about lighting, heating methods, floor tiling (whatever), guttering and downpipe drainage.
Remember that the conservsatory needs Building Regs approval if you are proposing to remove the existing (external) doors away between the house and the new structure. You may also need Planning Consent.
Ask these questions to your shortlisted companies.