A shotgun mic is always a handy item to have in your arsenal. However you may actually find that in noisy environments the lavalier works better because it can be clipped unobtrusively much closer to the interviewee’s mouth than you would be able to get with a shotgun (unless you don’t mind the shotgun being in the image, or shoot really tight close-up interviews).
By the way, I’m presuming you’ll be hand-holding the mic rather than mounting it on top of the camera because the latter is universally awful for picking up anything other than background noise unless you’re standing right next to the interviewee and in a quiet environment.
In your price range, I can’t recommend anything at the lower end. You really do get what you pay for, especially with mics. So you’re probably looking at the Rode NTG-1 (£110 plus VAT) or the Audio Technica AT875R (also £110 plus VAT) both of which get excellent reviews.
However – and this is critical – both of these, like many pro shotguns, require 48V phantom power which the G30 does not provide so you’ll need an adapter box. See my next point on this as well. You will also want to get a pistol grip to hold the mic with. Sorry, the bills don’t come cheap for decent audio!
Beware that since pro shotgun mics are mono with a balanced XLR connection, whereas the G30 has a stereo unbalanced input, you can’t, generally speaking, simply plug a pro mic straight into the G30.
It is possible to wire an XLR to 3.5mm cable but you can’t just connect the pins, you have to drop one of the connections from the mic, otherwise you get balanced audio going into the stereo input and this can cause total signal loss when played in mono – it’s a bit of a minefield.
The best solution is to buy a Beachtek or JuicedLink powered XLR adapter box as this performs a correct conversion and provides the required 48V phantom power. (If you pay a little more you can get a Rode NTG-2 which can operate off an AA battery but you’d still want the adapter box to do the balanced to unbalanced conversion)