No one cares if you call it RT phraseology or RTF everyone knows what you mean.
OP - You want to ask for a cloud break decent -
"<station> request cloud break descent"
"<callsign> descend to altitude xxxx, QNH/RPS xxxx (xxxhpa)"
"xxxx (xxxhpa) set, descend to altitude xxxx <callsign>"
They will get you to the TSL (terrain safe level) that will be on their radar screen which will depend on the area (1000' above the surface level or to 2000' over mountainous terrain, rounded up to the nearest 100'). At that point if you require further (which you will need to ask for) they will automatically put you on a TS, remind you of your own terrain clearance and then approve your descent. They will not give you levels below the TSL. Any further questions, please ask. I deal with this request by civil and mil aircraft on a daily basis
Not sure this is what you will get - outside CAS you will not get a clearance, they may give a deconfliction service and they will probably advise the MSA but the rest is up to you.