A fortnight ago, on my way to the French Alps, and with a rucksack full of camping equipment, including a knife, but no gas or other fuel, I was subjected to a full baggage search at check-in for the Eurostar. This is slightly embarrasing, because you are detained in full view of the other passengers going through security and took getting on for 20 minutes.
As a result, I missed my chance to get breakfast as I'd planned at the terminal, and although I was still in time to catch the train, I was slightly rushed.
Unfortunately, security searches are unavoidable nowadays. If you're camping, you will probably also need to carry a sharp knife somewhere in your pack. So be aware that there is a risk that you will be searched and allow time for it. Eurostar security seem to be stricter in this regard than security staff at most airports.
Yes. The security man asked what I wanted it for: to which I answered 'Cookery, chopping vegetables and meat' (although I might also use it for cutting many other things on camp of course). He then explained that the regulations allowed knives in baggage up to 3" blade length, non-locking. If he had measured my knife (a Petzl Spatha), he might have have found it's marginally above the permitted length (82mm instead of 76.2), but he didn't, and it's certainly non-locking.
To my great relief at the time, I was permitted to keep the knife and carry it on board in my pack.
Also worth noting that I had another knife with me: a Swiss army knife that was in my 'hand baggage'; this went through walk-on security without question.
And a final note regarding the attitude of the security personnel. They were not quite rude: I would describe them as being brusque and stern. My advice is that it is essential to control your temper in these situations, but that can be quite difficult to do, and I was certainly becoming impatient about it by the end of the process.
When I've gone through security, I keep kit likely to be of interest to them in a separate compression bag: cooker and knife and whatever you use for lighting. This means they can see straight away what's where and whether or not you are carrying gas bottles (which I think they will confiscate straight away, but not so meths AFAIK). If they still want to have a look, it's less faff and means you don't feel like you're being singled out as though you were a naughty kid stood up in front of the whole school.
Thanks, Man on Stilts, for all that info. I'm pleasantly surprised by the knife situation. It certainly increases the likelihood that I'll try Eurostar rather than flying before long.