The problem with Indian restaurants in the UK is that few are Indian. Most are Bengali or Pakistani.
However there are a few good'uns.
Chutney Mary at 73 St James's Street, St James’s near Buck House is a great Punjabi restaurant, but not a cheap choice. Instead a treat.
Tayyabs at 83-89 Fieldgate St, Whitechapel, is legendary and more affordable. Best to book as it is extremely popular. Their lamb chops are beyond legendary!
For authentic Indian vegetarian food check out Drummond Street just west of Euston Train Station. I'm no vegetarian but my London home is nearby and for many years I worked even nearer. Went there a lot.