I'm sure that Spyder Cars near Peterborough could do the conversion for you. They have their own in house paint and body shop and can fabricate the hood frame and other parts needed.
www.spydercars.co.ukChristopher Neil Lotus, used to be the purveyors of the most popular conversion for the +2, now seems to be the Oakmere Group
https://www.oakmeremotorgroup.co.uk/lotus/about it's possible that they could point you in the right direction.
Having had extensive work carried out to one of my own cars to make it a drophead (an Elan not a +2) I know how expensive it can be, so would try and point you towards a completed car that is priced at ?17,000 which is probably less than a conversion would cost you once you have paid for all the rest of the restoration.
No affiliation to the seller
http://www.carandclassic.co.uk/car/C829660