I have no idea whether there is a discount - you could ask the horses mouth, or at least one of the ATC's you intend visiting.
As for recommendations, I reckon that all the islands are worth visiting. When the weather is nice, there is no better flying in the UK, when it's not, there is no more interesting flying in the UK!
I believe (please correct me if I'm wrong), only Oban and Stornoway regularly hold 100LL, but may be available elsewhere by arrangement. Don't forget Broadford (Skye) and Plockton as well (Not HIAL airports).
Speak to the ATCOs at your planned destinations - in my limited experience, they are extremely helpful.