Which countries are you comparing the UK against? The USA is the easiest example.
In the USA, fuel is about 3-4 times cheaper, and that makes a big difference, especially on a very old plane whose investment cost was written off a long time ago and which is maintained to the absolute minimum standard (as most rental spamcans are).
Then there is a load of smaller factors between the USA and the UK. For example, typical Cessna/Piper parts are made in the USA and by the time they get here the price has doubled due to CAA-required documentation alone - higher still on small items like gaskets. General import messing about, the standard rip-off UPS/Fedex/DHL shipping costs (say £50 for a small package) and bonded warehousing don't help. On big things, I once paid £11k for a prop (with JAR145 paperwork) which in the USA listed for US$9000 - much of that 2x price difference was due to distributors having the UK market tied up and the US supplier not being able/willing to supply it with suitable paperwork.
But if you are comparing a UK rental price with e.g. a Swedish rental price then I can't help you
I know what it costs to run a new complex retractable though. If I was renting it out indiscriminately (like schools do), then the rental rate would depend hugely on the utilisation. If you can put 600hrs/year on it, you can rent it for perhaps 1/3 less than if you put 300hrs/year on it.
Things like insurance/hangarage play a very small part, a few quid per hour at most - but only if again the plane is utilised.