There is no direct, official, alternative to morning dress. However, most formal occasions in the UK that require morning dress also have an alternative as lounge suit or national dress. This is what HM The Queen specifies in her invitations to Trooping the Colour and garden parties. I am attending Trooping the Colour on 11th June and I did check with the Household Division that a kilt with dark jacket and tie is an acceptable alternative.

There are very few events when morning dress with no alternative is specified and for those, one should wear morning dress.