I would be tremendously cautious with issuing upfront payment to someone whom I have known only very shortly.
Besides, her story sounds utterly suspicious to me!
Someone who claims to have a valid visa to the UK, needs to show full proof of financial assets upon registering (the MINIMUM fee that they required, when I last enquired was £7.500). I cannot even faintly fathom how someone with such monetary backing would or could ask or even need help with booking RETURN flights. When I had my last invitee, we even needed to show the booked return tickets, so as to ensure that the guest wouldn't convert the temporary stay into a permanent visit.
If you care to follow my posts, many of them deal with the issue of scamming, but you're welcome to contact the admins to sound out the lady. The do an admirable job of eliminating bad eggs. To me it sounds all to fishy already.
My key questions:
1) how long have you known each other?
2) how old is she?
3) who is the 'driver' of your communication?
4) have you only texted or also spoken via camera?
5) have you seen any official documents?
6) have you connected via social media
(Show me who your friends are, and I'll tell you who you are).
7) has she offered you to visit her too?
  has she shown you 'around' - photos of her surroundings, where she lives, her place of work…
The golden rule of thumb is that meeting in real life requires trust, based on patience and good will.
The more people let you in, the more serious they are.