Penny Washers, have a look at Cap 410. Part B (at para 4) says that a FISO can give instructions to aircraft on a manoeuvring area, but these instructions are only valid up to the runway holding point. Thereafter the FISO becomes an information service only and cannot give instructions, but can request information as necessary to perform its function of:
"issuing information to aircraft flying in the aerodrome traffic zone to assist the
pilots in preventing collisions;"
This suggests you aren't required to "request" a backtrack at a FISO field as entry to the runway from the hold is at your discretion (as with a radio field), however the FISO can actively request information from you e.g. 'report lining up'. CAP 410 then also distinguishes between calls for 'entering the runway' and 'lining up' as two seperate calls which seems to make sense as you're not actually 'lined up' until you've reached the end of the runway and spun round.
This clearly differs from a radio which, as I understand it, cannot give instructions or request information under any circumstances and is essentially a passive listening only function, but which can pass details of known traffic and winds.
Happy to be corrected on this if it's wrong - the CAP docs are not the most concise!