I have a pretty ponies tweed jacket and absolutely love it- more so because RRP is £195 and I picked it up secondhand (near new condition) for £40!
I have really long arms and this seems to be generous in this department. Unfortunately they are in lancashire as they do a really good alteration service when you buy from new and gives a great fit but bit of a trek from kent! http://www.prettyponies.co.uk/levels.asp?level_id=168
I use mine for eventing, and mine is green with red and gold overcheck on it, I would have said absolutely fine for dressage I love their tweeds and think they look very smart in any sphere. I just looked at the musto one- now shower proofing show jackets that is a fantastic idea, the number of times you take waterproof off for a 6 minute dressage test and end up sodden, cold and miserable.
OK I am really thick. I am also looking for a new hacking jacket but I have to confess I do not understand the sizing! I would probably want a size 16 as I have wide shoulders and would like to be able to fit a BP underneath (I hate them over the top) - so what size in inches is that?