Steam's own refund
Steam will refund a game for any reason if you ask within 14 days of buying it and have played it for under two hours. You ask through Steam Support, and the money goes back to the payment method you used, or to your Wallet if you prefer. For a fighting game, two hours is enough to try the tutorial, a few arcade runs and a handful of online matches, so use them deliberately.
Add-ons follow a related rule: a fighter or pass bought on Steam can be refunded within 14 days if you have played the base game for under two hours since buying the add-on, unless the store page says the item cannot be refunded. Pre-orders can be refunded at any time before release, and for 14 days after release under the same two-hour rule.
The Consumer Rights Act 2015
A game is digital content under the Consumer Rights Act 2015. It must be of satisfactory quality, fit for purpose and as described (sections 34 to 36). If an update breaks online play for weeks, or a mode on the store page is missing, you can ask for a repair or replacement first, and if that fails, a price reduction, which can be up to the full price (sections 43 to 45). If the download damages your device or other files, section 46 covers that too.
The 14-day cancellation right, and how you lose it
Buying online normally gives you 14 days to change your mind. For digital content, that right ends when the download or streaming starts, provided you agreed to it starting early and acknowledged that you would lose the right (regulation 37 of the Consumer Contracts Regulations 2013). The Steam checkout handles this step, which is why Steam's own two-hour policy is usually the route that matters.
Card protection: chargeback and Section 75
Chargeback is a card scheme rule, not a law. If you paid by debit or credit card and the seller will not put a real problem right, your bank can try to reverse the payment. Ask within 120 days.
Section 75 of the Consumer Credit Act 1974 makes a credit card company jointly liable with the seller, but only for a single item priced over £100 and up to £30,000. No game or pass on our card costs that much on its own, so Section 75 rarely applies to a fighting-game purchase.
PayPal. Paying through a PayPal account puts PayPal between you and the seller, which usually breaks the Section 75 link even if a credit card sits behind it. PayPal runs its own Buyer Protection instead, and claims must be opened within 180 days.
No card fees
Charging consumers extra for paying by card or PayPal has been illegal since 13 January 2018 (Consumer Rights (Payment Surcharges) Regulations 2012). If a shop adds a fee at checkout for using a card, it should not.
Where to get help
Start with the store: Steam Support for Steam purchases, the shop's own support for keys. If that fails, the Citizens Advice consumer helpline is on 0808 223 1133 and can explain your options and pass the case to Trading Standards.
This page explains the main UK rules for buying games in general terms. It is not legal advice for your situation.