Asking for a FIS is fine, but on a busy day, it's not unusual to hear 3 or more GA pilots calling into (say) Luton for a FIS and being asked to stand by.
I've managed to get all the way from Sandy to Panshanger without making it out of the standby stack, after which I don't need the service anyway!
If you've heard a few standbys and it's sounding as if the controllers are busy, then I tend to give a FIS request a miss if I'm reasonably happy with the airspace, partly to lessen their load.
The new conspicuity (sp?) code around Luton/Stansted is great for this kind of issue.
Also, some of us are just 'floating around', no particular place to go, just enjoying the scenery. It feels a bit daft calling in for FIS with direction described as bimbling about!
BW