Usually when I travel to the UK I buy 3's "Internet with Legs" SIM card from Amazon which is preloaded with 24GB of data and just works as soon as I put it into my Huawei 4G mobile hotspot. I also bought a second SIM card at WHSmith at Heathrow T5 from EE which seems to be marketed by SIMLocal. Same thing, put it in and it just works.

Given how easy this was, I decided to buy SIM cards for our phones instead of just the mobile hotspot. I bought a 3 UK 50GB SIM card at Morrison's for 99 pence, and bought a 15 pound voucher. This was cheaper than the other SIMs I paid for. The packaging made it look like I get 50GB of data if I buy the 15 GBP voucher. I applied the voucher, received a message that I would be charged 5p per MB, and then following day I was asked to top up again since I had gone through 15 GBP or exorbitantly priced data.

This is the response 3 sent me after I complained on Twitter:

"This is an automated message, the top up credit value applies first then converts to the add-on if purchasing the pre-paid data pack. This relates to having just credit on the account without conversion to the add-on (Data Pack) for 30 days."

So, how I do convert it to a Data Pack? There is no information about this on the packaging and even in the portal. It feels like an elaborate scam. I'd like to be able to trust the system when I can't pre-purchase a SIM card from Amazon or Heathrow.

It seems you have to use the mobile app to apply the data pack to your account. When I used 3 maybe 15 years ago I remember I had to send a special sms after topping up credit to convert the credit to a data pack.

https://www.three.co.uk/support/new-...ing-data-packs

During the call where you apply the voucher there is an option to use it to buy the data pack, rather than just leave it there as payg credit.

Just listen carefully to the voice prompts

Thank you both! Googling was not helpful, nor was the Three website, but your advice made it clear. Basically, if using a store bought 3 PAYG SIM card as a non-UK resident, set up everything by using the SIM in a phone, calling 444 and following the prompts. That was the only way to have access to all of the options. I could not find the special SMS code anywhere (EE makes theirs obvious) and neither the app nor the website offers a conversion option (it's expected that the customer buy the data pack during the introductory 150-200MB provided with the card, when money isn't deducted).

Far from intuitive and it basically means that the GB and GBP price printed on the product is meaningless because the customer is given the option of which pack to buy when they call.

UK mobiles are among the worst in the world in my opinion. I have had top up problems with several UK PAYG Sim cards. Data is often very slow. The worst thing though is the coverage. I haven't found a UK Sim card that works nationally. The coverage maps also lack any detail so you can't plan. Sometimes you just have to go and buy another Sim and top up to make a call.

If you can find a public telephone box it is broken, more often than not, and normally always reaks of urine. Why can't they clean and maintain them?

This trip I decided to get two SIMs because of where were traveling: 3 out of habit, and EE which the map showed had better coverage in the Highlands of Scotland and the islands.

I've had a decent time with coverage and speeds (4G) but not sure how it works elsewhere in the UK. Three/3 UK was decent throughout most of the country, except for the islands signal was non-existent and EE was quite fast. In general, I found EE to be more reliable, with better speeds and coverage compared to anything in the States. The SIM gave us great 5G in the cities when used in a phone. Over here, on Verizon which is one of the better carriers, it's nearly impossible to get 5G anywhere, even in cities.

Despite my frustration with Three/3, I will give them credit on one thing... free roaming in EU (Holland in my case). I was limited to 12GB which wasn't great, but connection was solid on KPN. EE wanted to charge me per day to use the last 2GB on my card.