I don't know what is best, but I recently bought an unlocked International HUAWEI E3131 UNLOCKED Mobile Broadband Dongle 21MBPS 3G 4G SIM FREE Dongle for my Dell Studio Laptop. (Although my Dell Laptop has a SIM Card slot underneath the battery, it doesn't seem to have the hardware installed to match it). Anyway, I was in central Scotland and needed a WiFi connection.
I bought that on eBay for UKP22.00. Prices in a very large local TESCO were stupidly high, so eBay was a no brainer.
I bought an EE Data Card 2GB in Edinburgh 2GB for UKP15. It can be topped up and it is 4G, but I think the data allowance is lost if it is not used in the 30 days or so. I think they probably allow you to buy more on line using PayPal, Credit Card, etc.
Haven't tried it elsewhere - I think if I was in Norway, etc. I would just buy another card there. I guess if you have a data card for an iPad, etc. with one of the big providers, they will have partners in most other countries and your UK supplier may supply a second card at a better rate.
As regards performance, it worked OK, but in Central Scotland/Fife at my Mother's house, it was difficult to get 4G speed, because the mobile signal just wasn't up to scratch. It was however, much better in Aberdeen at a sister's house. The Dongle has different coloured lights to indicate what signal capability is being received.
Sorry no experience of other Mobile Broadband dongles and packages in the UK.
Grey Beard