Why couldn't you empty the potty in the pub loos? Are they innaccessible with a toddler i.e up or down flight of stairs, and you couldn't carry child and potty safely?
The folding Potette plus looks good, with the disposable bags. Or you could line the current potty with a pedal bin liner and a few sheets of kitchen roll folded into a pad inside, or a cheap sanitary pad to soak up any wee, and tie it up after use, dispose at home/dog poo bin/ nappybin. The bonus with the Potette is it is smaller and can be folded, also can be converted to a toddler toilet seat.
I'm carrying a big potty round everywhere this week (day 5 now) and have been using a nappy to soak up the wee afterwards - just turn it inside out first and it soaks it up in seconds. Nappy into sealed bag and wait to find a bin. Not ideal to take potty to library/museum/shops but DS can't hold on yet, and won't sit on a toilet.
We have a my carry potty DD1 has only really started using it the last few days, but loves it & it really does seal (I tested it by filling with water & shaking!) so no worries about leaks if there's nowhere appropriate to empty it!