You don’t need an architect, any experienced builder should be able to do such a job without spending money on an architect.
For a proper loft conversion you will need to engage with building control, there will probably be structural calculations required – I would suspect you will struggle without any technical drawings.
An architectural technician is what you would require, not an architect. Pete Walker does a lot of work on the island – we used him and he is good. Or there is a firm in Portsmouth (Thorns Young) who I have used recently and have been impressed with – they a fast, reasonably priced, and they do all of their structural calculations in house which speeds up the process.