You may need to justify good reason for possessing one in a particular place:
Wandering around the street to look ‘cool’ isn’t, but travelling to or from a site with it appropriately packed is.
You require a defence to purchase, and any offence would be committed by the seller if they sell without reasonably establishing your defence.
It’s up to the seller how they establish that.
The most generally recognised defence is the UKARA scheme, which links to membership of participating sites, but also requires you to be aged 18, show ID and play 3 times in two months.
If you have field membership that the seller recognises then yes.
For example you could sign up as a member to a UKARA site on your first day, but they would track the game days until you qualify fully for UKARA by completing the required number of games.