Victoria vs St Pancras Int'l

Because they can. It's called capitalism, what the market will bear.

The Gatwick Express is faster (non-stop, 30 mins.) and aimed at airport Travellers. The Thameslink trains are for commuters, with many stops, including three central London stations (Blackfriars, Farringdon and St Pancras) and a lot slower (50-60 mins.). It is actually slightly quicker to St Pancras to take the Gatwick Express, and then the tube, than it is to take the direct Thameslink train, but a lot more effort as you have to lug your bags up and down tube escalators.

I don't think you can pay the difference. But getting off at Blackfriars and taking the tube from there may be the best option, depending on which tube station your hotel is nearest.

Why? Well the Gatwick Express takes about 30 mins to get to Victoria, while the Thameslink train takes about 60 mins to St Pancras. Plus it has express in its name and targets visitors from Gatwick airport who have money to burn.

As you’ve already bought your ticket you could use it and get off at Blackfriars, then take the tube to Victoria.

Or you could apply for a refund https://www.thameslinkrailway.com/tickets/buy-tickets/refunds-and-changing-your-ticket

then buy a ticket for a Southern train from Gatwick to Victoria which costs from £12.00 and takes about 5 minutes longer than the Gatwick Express.

http://www.nationalrail.co.uk/

£19.90 is the price of an anytime single on Gatwick Express.
The price of an anytime single NOT on the Gatwick Express is £16.20. This is on a Southern train, not Thameslink, which doesn't go to Victoria.

If you use contactless payment or Oyster and not travel on the Gatwick Express it is £14.70 peak and £8.30 off-peak.

(You haven't said at one time you are travelling to give help on whether you really need an anytime single. EDIT - the fare above of £12 fare is an advance which is valid on one day, whereas the Anytime Single is valid for two.)

£10.70 is an anytime SINGLE (not return as you say above) from Gatwick to St Pancras (not an unspecified London terminals). You haven't stated from whom you bought this ticket. Refunds may be charged a fee, typically £10.

This might sound complicated, but in essence you have bought a ticket valid on one company, and now you wish to travel on another.

Thanks for the information. Since I've already purchased the Thameslink tickets, I'll take the tube from Victoria Station to St. Pancras and Thamelink from there.

On the Thamelink St Pancras to Gatwick, I did not pay 10.70 GBP. I paid less than that, can't remember the exact price but it was cheaper than the 10 GBP.

I used the Oyster card to get from St Pancras to Gatwick on the Thameslink. I much prefer staying in a B&B in the Kings Cross/St Pancras than at Victoria Station, ie numerous B&Bs to choose from at KC.

The cheapest fare between St Pancras and Gatwick is £8.30 (off-peak Oyster PAYG)

When I booked the ticket, it indicated I had the cheapest ticket available.

The above isn't available as a paper ticket though.

I believe the 8 GBP figure is correct. It pays to have an Oyster card if you intend on going back to London, which in my case is a given.