There is no 'best' portrait lens, there is only the right lens for the particualr portrait you want to take.
People go on about c90mm being the 'ideal' focal length for portraits, but that only comes form working in studios and the distance from the photographer to the subject.
You can shoot portraits on any focal length, you just have to choose the right focal length for the shot. If you want to put the subject in context with their environment then you'll want to use a wide angle. If you want to concetrate everything on their face then a mid-telephoto would do. If you want to really compress perspective to bring a distant background closer then a long telephoto would work.
Having said all of the above, for a budget of around £300 I would look to either a Sigma/Tamron/Tokina 24/8-70/75 f:2.8 lens or a Sigma 50-150mm f:2.8 as being the most flexible depending on the type of portraiture you'll be doing. The f:2.8 max paerture will allow you to control the depth of field enough to isolate the subject should you wish to. For either of these you'd have to be looking at the 2nd hand market, and even then you might not find the 50-150mm f:2.8 at your price point.