Based on the clothes and the decor I suspect he's having tea at The Ritz or one of the other flash London hotels that have a dress code. And where afternoon tea is expensive, like £80 a head, more if you have champagne.
Your guess is correct, this is indeed the Ritz.
Afternoon tea (the fancy one, I'm not going to sperg about the differences between afternoon tea and high tea) is served in the Palm Court:
There's a champagne flute on the table, so they've not gone for the cheapest option (which is traditional afternoon tea). Ironically the traditional tea is the "classy" option, which is a selection of teas, fancy little sandwiches, scones with jam and cream, fancy pastries and tea cakes. They've opted for a "bubbles" tea which is a concession the Ritz have made for people who are willing to pay for the Ritz but are not willing to behave like they're supposed to be in the Ritz and want a single glass of champagne because they're already spending so much money on being there and thus they need to "celebrate", even though champagne defeats the whole point of trying all the fancy teas they have. It would have been about £95 per person ($120). Afternoon tea is normally served at around 4pm although they do other sittings.
They're then in the Ritz Restaurant.
While Ritz diners would normally go for table d'hôte (fixed price menu of multiple courses), you can of course go à la carte or even try arts de la table and dine on Beef Wellington (about £180/$225 dollars, and that's if you just get a beef wellington for main with no starter or dessert or wine with your meal).
Regardless of how "ironic" Hasan was being, he either has spent a day mooching about the Ritz (presumably stopping over at the Rivoli bar for cocktails between afternoon tea and his dinner reservation - maybe he went for their signature rusty nail cocktail of Glenmorangie Signet Whisky with 1960’s Drambuie - £90/$110 a glass) or he was staying at the Ritz, because if you go there for afternoon tea you don't just get to hang around for a while afterwards without paying for something.
So he's spent a hefty sum of money. Even if he's pretending to be lol fake royalist (I honestly doubt he is - Britbongs don't see the coronation through the same lens as the rest of the world, he's a "third culture" person who also was apparently visiting a wealthy Brit, I fully believe he's on board in the same way Ottoman ambassadors who stayed at the Ritz were on board) it could easily be in excess of £400/$500 (and that's if he showed restraint and didn't decide to splash out on the five course dinner and a couple of bottles of Château Mouton Rothschild - you know he has the money) that he's spent on afternoon tea and dinner on one day for his fun little holiday in London. And he's a "communist".
I thought the $200,000 Porsche was the most egregious thing he could do, but there's something about the Ritz being so archetypal old world aristocratic - it's like he wanted to be the opposite of Communist in a way that'd be visually put him in the same class as the Tsars instead of the Bolshevik proletariat.
