Best way to convert One4all gift card into a steam wallet card?

Simple. Go on the One4All website and swap for Currys e-gift card. Use the Currys giftcard and purchase Steam Wallets from Currys website via CompleteSavings and also get another 10% cashback. For some reason they pay cashback on this.

I bought 5 or 6 Steam Deck using this method and saved 30% by saving my monthly One4all from Completesavings (my family had 3 accounts) and the 10% cashback.

This was back when they were still selling £50 giftcards. Got a bit annoying when they only started selling £10 and £20 cards with a limit of 5 of each per order, but you could make several orders at the same time. As a precaution it is advisable to order for click and collect. Just imagine if the courier said the item was delivered but in actual fact it wasn't OR if they only sent you 4 instead of 5 giftcards. It is probably best to collect and check everything before leaving the store.

Just checked the Currys website and they don't have any steam wallets in stock currently. You could purchase in store but without the 10% cashback. I had an argument with them instore around 6 months ago when they suddenly decided that they wouldn't accept One4all for purchasing Steam giftcards. Said it was a new policy. I wrote a letter to head office and they confirmed in their letter with a £10 giftcard apology that it is accepted. After showing them the letter they admitted they made up that new policy so that they didn't have to deal with One4all being declined when customers were unsure how much credit was left on their cards so you need to check your balance before purchasing. (edited)