I have experience with this issue last year.Whilst in Toronto Pearson Airport,I had previously been in the USA and bought a leupold scope wrapped it up nice an safe in my hand luggage.
whilst I was going through the Xray my luggage was identified and I was stopped,They asked me to open up my bag and declare what I had,they stated that my scope was a firearm!!!
I argued it was an optic,but due to their strange laws they declare that it was part of a firearm.
I explained it was a small telescope therefore cannot be used to discharge a projectile.classing it as an optic.
this disagreement went on for sometime and they told me to place it into my suitcase ,which had already gone through in check in at the desk.
I was left with 2 options
1 leave it and walk away or 2 stand my ground,after confirmation with the security manager,they decided I could take it on the plane.
so be careful bringing into the uk any scopes
Good luck