At the cost of 30 day's rental, I can buy a lens !
Wrong.
For the Canon 70-200 f2.8 II IS quoted above;
30 days is charged as 5 weeks. 5 x £91 = £455
Price for a new lens in the UK = £1869
Even B&H sell it for the equivalent of £1310 in the US ($2074)
Even if you paid top UK rental prices from the biggest hire dealer in the UK (Fixationuk.com) @ £140/wk it would still be cheaper than buying.
Lensesforhire.co.uk seem to offer a very good deal.
And yes, lots of things and services cost more in the UK.