I've done that, but my boat is only 23'.....
The lower Grand Union towards the Thames isn't a great place for mooring, unless it's changed greatly in the past 6 years. The basin at Brentford is fine, and you're near shops there.
Lots of places to moor on the Thames, but often time limited. If you have time, go down the Wey Navigation and the Basingstoke Canals. Well worth it, but mooring is not quite as easy, and again time limited.
Loads of places to stop on K&A, but be sure you have a gangplank, or be prepared to jump! Many folks down there have a hole in their plank so they can pin it in place, as it's hard to get close into the side. I went all the way to the Bristol Floating Harbour. It's such a great canal for Kingfishers!
Have a great trip!