If you are 18 or over, you are legally fine to purchase, or attempt to purchase, a realistic imitation firearm from anyone in the UK. You are not committing any offence by doing so, and you don't need any sort of defence or licence to buy or own it.
It's the seller that's committing an offence by selling it. They need a defence to do it, one of which is selling for the purposes of historical re-enactment. They clearly don't believe that's your real intention, they're just asking for a MVT number that they can record against the sale in the very, very unlikely event that it ever becomes an issue for them. Some sellers even accept "defences" that are (legally speaking) nothing of the sort, e.g. cosplay.
That's a concern for them, not for you. They want to sell, you want to buy.
My concern would be what you intend to do with it, and whether you're likely to attract the wrong sort of attention. The double-post isn't a good start.
Please be aware that public possession of any imitation firearm,
realistic or otherwise,
is an offence, and that you need to be able to produce a reasonable excuse on demand. That would mean being on the way to or from an airsoft event at an insured site,
not showing it off to your mates down the local woods.
tl;dr version - if you're at least 18, you're fine to buy and own get an assault-style toy inside the UK. Just be very careful what you
do with it.
