In reply to Blue Straggler:
They look legit. They are an Australian company so they might be finding flights which are priced for the Australian market and selling them in the UK.
From the look of their website, i doubt you'll get royally screwed but don't expect any customer service if anything goes wrong. From the airline's point of view you are JetAbroad's customer. From JetAbroad's point of view, you've paid you money and anything else is a cost they don't need.