Strippers Paint Removers has a product called Kling-Strip which could remove oil based gloss paint from your radiators. If you contact them directly, they can put you in touch with several specialists in your area who might be able to undertake the restoration work for you. The inside of the radiators can be cleaned with a Powerflush or chemical cleanser. This job is best left to an expert plumber in your area who specialises in installing central heating systems.