longfinal
Chilli monster's advice on RIS is absolutely spot on but I think your question highlights certain misunderstandings between ATC and GA....you can 'demand' a RIS if you like but if workload is too high the controller doesn't have to provide it. The key is communication..if it's got a bit hazy/busy request RIS and tell the controller why...eg some months ago, whilst routing via BNN, Luton Approach refused RIS and gave FIS...due workload (the freq was quite busy)..however when the weather got murkier still a few miles on I reported "G-XX IMC, requesting RIS when available" which was immediately sufficient to upgrade the service. (Thanks Luton)
If in doubt, shout I suppose!