How much does Professional Indemnity Insurance cost in the UK?

letting what, flats? construction equipment? airplanes? It matters a whole lot.

How many will you be letting? What are your estimated gross receipts? All that matters. "Professional" is one type of coverage, "general" is another. Most won't sell you professional until you also have general. I strongly suspect you don't need professional at all, only general. If you're letting one flat, it should cost about 500 pounds a year, minimum. When you have more, the rate goes down. If you're letting to a business, or letting construction equipment, the rate goes up.

I wouldn't even guess how much it costs to let an airplane, thousands and thousands. So all these "guesses" are just that, and won't do you any good. You'll have to get real quotes.