Let me say the generous thing first: dining alone in London can be done beautifully. The city is full of counters built for it — a seat facing the kitchen, a book against the water glass, staff who treat a single cover as a guest rather than a gap. If you travel often, it is a skill worth having.
Doing it well
Choose the counter over the table; the kitchen is company of a kind. Go early, when the room is gentle. Order slightly too well — the single diner who orders properly is treated royally, because kitchens notice who is paying attention. And take the book, not the phone. A phone at a table for one reads as waiting; a book reads as choosing.
What the counter cannot do
There is a moment in every good dinner — the second glass, usually — when something lands on the table that wants an answer. A remark, a look, the plate itself. The counter absorbs it politely and gives nothing back. This is the honest limit of solo dining: the food is complete, and the evening is not.
The rooms know it too. However well they treat one cover, every table in London is laid for two. The geometry of the whole institution assumes a face opposite.
The alternative
The difference between a good dinner and a good evening is rarely the restaurant. It is whether the conversation deserved the room. London does its part every night of the week; the seat opposite is the variable.
You stop being the one dining alone the day you decide to. The asking is the easy part.