I’ve been asked what’s in the bag once, and not the other time, both from Gatwick.
But both times I’ve had a under the limit hold bag as awell. I suspect they’d maybe enforce the ‘no extras in the bike bag’ rule if you turned up with a bike bag and nothing else.
In terms of spares I just take the normal camelpack stuff plus a few brake pads, gear cables, and one spare tyre. I did originaly pack everything down to bleed kits and BB-spanners, but took it all out as although it was under the limit, it’s a holliday, not a challenge to see if you can man handle 50+kg though the airport. And even on a weeks holliday, it’s unlikely to be over 200miles riding, would I take half the crap I originaly thought of with me on a comparable trip to the Lakes or Scotland?