@Desiringonlychild - good question.
The like for like data of prices of things in London versus outside-London shows that London is more expensive. A long time ago, I was told that you can tell how expensive a place is compared to other parts of the UK by the cost of a pint - it's a good litmus test apparently lol
. Just did a google search and London comes on top, followed by other places I'd consider affluent too (Oxford, Edinburgh etc) - www.savethestudent.org/news/the-cheapest-place-to-buy-a-pint-in-the-uk-revealed.html
It seems like it's the knock on effects that make London more expensive for people. So, rent/land price is higher, meaning a business needs to cover their overheads through their products being a bit more expensive than if they had a business outside London etc.
Both of us are only talking anecdotally, of course.
You've given some great examples of free or cheap things going on, but sometimes (not always) you do get what you pay for. Free museums/art galleries are great - no doubt about it! However, the extra seasonal exhibitions that you have to pay for are often worthwhile to get a more diverse experience/understanding (and they tend to be time limited, unlike the free exhibitions, too). Those extras can feel culturally richer for some people and worth paying for.
As for the £10 per person Indian place near Euston (sounds fantastic btw - what are they called?) - it might not be everyone's cup of tea etc. Then again, there are those vouch codes you could use for the chain restaurants that can feel like good deals - but even then some of the London restaurants are excluded from those voucher deals because they're prime locations.
In some areas of more central London areas they don't have many of the big supermarkets (land being at a premium) so local residents can end up spending more in the convenience Express/Metro/Local stores. The extra pennies add up. It depends on which part of London you are in - it's a big place after all. It's a bit better now with online shopping though.
I think it boils down to the London lifestyle being more expensive; having more things to do can usually mean a more expensive lifestyle. My family who live in London like to be able to fit in with what's going on and do social activities - West End theatre etc. So, it's the London lifestyle that can add up more so than other places, simply because other places don't usually have as much going on as London. And it's part of what makes London a great place to live - loads to do!