The model 27 nameplate was used for over 80 years, and was applied to 3 different “families” of designs, though all were similar in size and power. “Diana” is of course the trade name of the Mayer & Grammelspacher factory in Rastatt, Germany, one of the world’s largest and oldest makers of spring-piston airguns.
Your description matches the first-generation guns made from Great War days up until the early 1930’s, a very nice and well-made collectible. It’s a simple design with leather piston and breech seals, basic open sights, and a single-stage trigger with an adjustment for sear engagement (if the gun is complete, there will be a rather homely double-headed bolt sticking out the front of the trigger guard, this is the trigger adjuster).
I don’t live in the UK so can’t speak for local parts availability, but obviously components for such an oldster will not be common. That being said, the main perishable bits (piston seal, breech seal, mainspring) can be made or adapted easily enough.