There's not really a great deal of difference between Harpenden and some of the local games I used to attend when I was growing up in Scotland; Aboyne, Ballater, or Stonehaven in the Deep South, etc.

In London the Highland Games at Richmond have been revived after a lull. Another well known English game with comps is Ashbourne in Derbyshire. Also in Derbyshire is the Chatsworth Country Fair Pipe Band Contest.
The Colchester event in Essex is another biggie in the piping calendar, as is Stotfold in Bedfordshire. There are quite a few others, I'd have to draw up a list to make sure I got them all.
There's a strong piping scene in the NW of England too, and a fair bit of action on the South Coast.

There are a fair few non-competing bands in the London area, but competing bands registered with the London Branch of the RSPBA include;

Grade Two
The University of Bedfordshire Pipe Band
Grade Three
The Pride of Murray Pipe Band
The Pipes and Drums of RAF Halton
Grade Four
British Airways Pipe Band
City of Norwich Pipe Band
City of Plymouth Pipe Band
Epping Forest Pipe Band
Essex Caledonian Pipe Band
Gatwick Caledonian Pipe Band
Glenduart Pipe Band
Glenmoriston Pipe Band
The Grampian Association (Corby) Pipe Band
Jersey Caledonian Pipe Band
Reading Scottish Pipe Band
Rose and the Thistle Pipe Band
Surrey Pipe Band
The University of Bedfordshire Pipe Band
The Welsh Piping Society
Cambridgeshire Caledonian
City of Rochester Pipe Band
Colchester & District Pipe Band
London Scottish Pipes & Drums