I'd recommend buying any Turkish Lira you'll need for your holiday in Istanbul itself. The best quotes are available at the currency changers outside the Grand Bazaar, with the Sultanahmet kiosks a very close second.
Avoid the booths at the airports, unless you must, the prices at Istanbul's Ataturk airport are slightly better than those you'll receive at London City airport. The curious thing at the latter is that the Travelex booth you see before check-in offers a slightly worse rate than the Travelex booth in the waiting lounge.
The post office in London was the best price available in the city and was definitely more friendly than the other options.
Thank you Turkey; for issuing tourist visas on-arrival to Indian passport holders (with an UK visa). Considering I made last minute plans for a holiday, I was basically looking at countries that didn't make you jump through too many hoops for documentation.
I arrived at Istanbul airport with some trepidation, never having travelled internationally without supporting documents. I need not have worried, the Turkish authorities were amazing. All I had to do was walk up to the visa desk and hand over my passport, the officer scanned the pages, found my UK visa, attached a Turkish tourist visa stamp (1 month single entry) on a nearby page and asked me for 15 Euros, no other questions and no harassment (if you travel on an Indian passport, you'll know what I mean). The entire process took under 2 minutes. One is required to have a return ticket and hotel reservation documents handy, so i'd suggest carrying these but they didn't ask me for them.
If you're thinking of going, start packing already.