The 416 is an incredibly popular platform at the moment and is in widespread use across many services/forces, if you're stuck for inspiration then I'd suggest googling the 416 as
@Druid799 suggested.
It's also very difficult to provide any recommendations when you fail to provide any preferences or even so much as a budget. As you're only criteria is "what is available" people could easily suggest optics costing in excess of £1000.
One of the best options on that market at the moment would be the optics produced by Vortex. You can find these for sale at
Tactical-Kit.co.uk and
Patrol Base along with a variety of other retailers. I would suggest ensuring that you purchase from a legitimate retailer when it comes to more expensive optics (read; genuine/real steel) as there are a lot of reproductions floating around the market. Vortex also have the advantage of offering, quite possibly, the best and most comprehensive warranty on the market which is also a lifetime warranty. It's worth reading up on if these are within your price range.
If you're after something cheaper then there are a variety of sights and scopes available from almost every airsoft retailer, amazon and ebay but product quality will inevitably vary. For one of my builds I needed a T1 Red Dot which would fit on a particular mount and as such I took the plunge on a G&G T1 Red Dot (I use the phrase plunge as there was no guarantee it would fit) and it's a good quality optic for the money.
The sight/optic debate largely falls down to personal preference in airsoft. The ranges you'll mostly be dealing with are well within acceptable distances for your normal vision and thus iron sights (if they work for you anyway). If you do decide to pick up a sight then, at least for the Vortex, it would probably be worth trying to zero it as close as you can but this is difficult due to issues airsoft guns have when it comes to accuracy.
Lastly, I'd recommend avoiding the magnifiers
they're really not worth it, beyond looks in my opinion. If you're adding it for looks then be aware that you're adding weight which I'd question whether or not is necessary.
Hopefully this helps a bit. Welcome to the forums and the community, if you get stuck then don't hesitate to ask but try do the legwork before hand.