I think if you're planning to keep a car for 6+ years it may be worth it. My first 250TD had retrofit a/c fitted by the previous owner. I got 10 years out of it with no costs
Retrofit a/c will cost about £1500 - or £250 / year over 6 years. There used to be a company in the West Country (Malvern?) that did it. I expect Di Avia in London would do it too. They used to convert lots of lease cars for Lex Leasing in the 1990s because it saved paying Special Car Tax and VAT on the installation