Only two rules.  Dress like they do. Dress simply. 

The English are conformists.  If you play golf in jodhpurs, if you turn up for a regimental dinner in shorts, you will shock and sadden them.  But you’ll shock them even more if you have the bad taste to be overdressed. 

None of your clothes should be over-tailored, nor your shoes over-new.  Miss Jane Harrison, in her Reminiscences of a Student’s Life, described the pleasure that she felt watching the Duke of Devonshire, at Cambridge, receive an honorary doctorate with his socks showing through the holes in his shoes. 

‘Right down to the holes in his shoes,’ she says, ‘I recognized that he was truly a Duke.’

Don’t think that in London you must dress as the English do when abroad.  In London, an Englishman ceases to be abroad.  Imitate him.  Dress as if in Paris.