Hello,
This is my first post on here but I've been lurking around in the shadows for some time...
Perhaps I could offer some advice from the other side of the fence on this one. The term "reasonable excuse", as has been mentioned countless times on this forum, is down to the person in possession of the knives/articles to prove. The only proof that I would accept as reasonable excuse (bearing in mind I can tell the difference between a £10 rubbish lock knife carried for the sake of it and a £100+ handmade knife) would be where you were going, how you were acting, and what else you were carrying. If you've got a bergen full of camping gear and you have a knife in the Bergen, in a place that you can't get to without decanting the rest of items, I would be more satisfied than if you had a knife on your belt or in your pocket (after all, when was the last time you needed quick access to your knife on public transport for a legitimate reason?).
The same goes for your behaviour. If you're upfront about what you have on you and offer your explanation, it's a lot better than being searched and the police finding it on you, when you've not told them about it. I guarantee when the police find knives unexpectedly, the experience will not be one they enjoy, and you're likely to end up in a pair of shiny bracelets at least until they're satisfied to let you go, or worse, take you with them.
Also remember, you may be allowed on your way, but you have to leave your knives behind. This may not be a huge issue for you if it's an easily-replaceable £10 mora, but I have knives that mean a lot to me, both in monetary and sentimental value, and I certainly wouldn't want them confiscated for any reason.
I don't work for the BTP, but they are goverened by the same laws as every police force in England and Wales, with the addition of the railway bylaws. If those bylaws say you can't carry knives, then that's that. All the police have to prove is that you had the article with you in a public place, and that it was either over 3" and locking/fixed, OR that it was made/adapted/intended for use as a weapon (REGARDLESS of the size of the blade or whether it locks or not) Unfortunately with today's opinions on knives, many people automatically deem them to be weapons, so it'll always be an uphill struggle if you have one on you.
Please don't take my words as gospel, as I'm sure there are some inaccuracies, and I certainly don't speak on behalf of every police officer in Britain. The only advice I can give is make sure you have a valid reason that you can back up for carrying anything sharp/pointy in public. YOU have to prove you're allowed to carry it, not the police.
And remember, if you're drunk or acting like an idiot, your excuse will not sit as well with the police. You could end up being arrested because you're not in a suitable condition to be in charge of the items, for for other offences. This is common sense of course, and I do not mean to offend anyone with anything I have said.
With regards to the sentencing, I feel that that was a kind of scare tactic, and was aimed more at the people who carry knives for criminal reasons. However, if you do end up in court for it, expect to be punished to that extent, as it may be that if it's in the news about a stabbing or kids with knives, you'll be treated as a scapegoat by a court that wants to justify it's actions against knife-carriers. Giving the excuse that you paid £100s for your tools isn't enough, and you'll just be £100s out of pocket when the courts order the destruction of the items.
I hope this has helped, and if there are any inaccuracies/questions, let me know and I'll do my best to answer/edit.
Cheers,
Luke