Don't plan on the use of anything as a form of weapon, self denfence or otherwise.
if someone follows you or trespasses on your land then that is intimidating but not sufficient for you to attack.
You can however fight if you are in fear, they do not necessarily have to make the first punch (but if they have been following you and doing nothing then that doesn't support your fear to use force as self defence, you could have to prove the accumulative fear)
Airsoft, BB, paintball, and air guns are definately not suitable for self defence. Unless it's a springer then you have to 'make it ready' with powering up, gassing up etc and loading it. Unless you are maintaining it at that moment there is no good excuse for having it ready to fire and loaded
There could also be a seperate offence with a £1000 fine. From 2011 there became the requirement that any air weapon is to be secured if minors could have access to the property. This was brought in because of a number of cases of children being injured from unauthorised use of their parents or families air guns, including at least one accidental death. The Home Office & police recommend weapons safes, but it is suffcient to lock them away. It's a stretch to include airsoft, but like paintball there are various parts of firearms legislation that we drop into.
Your parents could conceivably be caught out on this if you get out your airsoft guns to shoot or intimidate someone with you being too irresponsible to be in charge of them at home. At the very least your followers can just call the police to say you threatened them with a gun, at worst they can not attack you in fear of their lives because you have a gun. If they know it's just airsoft then they can forget that in the heat of the moment or not have any fear but just beat you up
Tell someone you are being intimidated, consider what you can do to just defend yourself in the moment, if you're a girl or like your looks then pop out your hairspray, perfume, aftershave, antiperspirant, sun spray etc and spray in their face (no need to light it into a flamethrower .... that's no longer reasonable)
Note that reasonable force does not have to be absolutely proportional. You don't need the right level of weapon to meet the force of the attack, you just need to be reasonable for the fear you were in at the moment and using anything around you to protect you. Don't use that to mean you can put anything beside your door so that it is to hand
Idealy stop something from happening in the first place. They are following you to intimidate you, don't be intimidated.
Are they going to attack you or do they just think they are hard because they can intimidate you and because you are responding?