"...one can get around barb wire fences easily on foot..."

Not in the areas I am familiar with, some of those fences seem to run forever. And many go thru ravines etc that are soooo steep I don't know how they put the thing there in the first place.

I have climbed many a barbed wire fence, wearing full uniform (including $150 pants), and never suffered a rip. Most fences, no matter what kind of posts they used, have a much heaver post (almost a RR tie on some) every hundred feet or so, also at corners. Go to that post, put one hand on the top of the post, and one foot on a strand at a comfortable level. Test the wire and its staple by putting some weight on it. Right next to the post it should be almost movement free. Assuming that it is, stand up on that wire, swing your free leg over, place that foot on another strand, test it, swing the first foot over the top, and down you go. Your gear can be tossed over first, lifted over or dragged under after you are over...

I would consider to it prudent to store bolt cutters or similar tools, along with many other tools.
Not certain its a good idea to put them in a BOB though, very heavy, and puts one at risk of arrest.
Breaking into or through someone elses property may perhaps be justified in an emergency, but remember the owner will probably have different views and even in the UK may be armed. In the USA it is probable that they will be armed.

Well, under these circumstances, a pair of bolt cutters could come in very handy.

I think the question is, do you want to carry bolt cutters, on the off chance that you might actually wind up in one of these scenarios, or use that weight/space in your BOB for more useful items?

At the fire company, we call our bolt cutters the "Master Key".

I used them yesterday to get through a locked front gate for an alarm system call.

Handy things, even the smallish ones.

Shifts into Alfalfa expresssion ( the Little Rascals) of my MPD. ' Oh give me land lots of land 'neath the starry sky above. Doooooon't fence me in......" I'll stop if a few members make immediate ETS online donations.

I always thought the "master key" is the gasoline abrasive saw I always see FDNY use - man, those guys cut anything with them - watching them take down roll-up steel grates is fun - or take down a section of iron fence. Saw a kid get impaled on the points of one of those steel fences outside a school - they took down a good sized section of the fence and loaded the kid in the ambulance with about a 3ftx3ft section of fence still attached - according to the news, the kid not only lived, but made a full recovery

I have a question for you, you being a fireguy. I have seen many a fireguy use that "master key" to open a locked gate, and they ALWAYS cut the lock, not a link in the chain. Sometimes very expensive locks. Is there a reason for that???

Probably out of habit people naturally go for the lock.

Locks are hardened...they usually pop once you get through the case hardening as it's very brittle compared to unhardened steel (cheap locks usually more so). Chains can be very springy and hard to cut through...you literally have to get the cutters through all of it.

I lock my motorcycle with overhead crane chain for that reason. It's elastic and the give makes it really hard to cut with bolt cutters.

Yep, crane chain is very hard to cut. I've said before I worked for an overhead crane company back in my salad days, and MOST of the hoists we sold were chain hoists (the thing going the lifting is the hoist, which rides on a trolly, which in turn is often on a bridge, which rode on the rails, and the whole thing is a crane) We used to get the chain delivered in cardboard 55 gal drums. The prefered method of cutting it was a big disk grinder (7 or 9" if I remember right), but we'd use the dinky 4.5" grinders in a pinch. Bolt cutters wouldn't touch it. If we couldn't use a grinder, a cutting torch worked wonders