The Hudl doesn't have 3G (or 4G), so you can't connect it to the mobile phone network (for which you'd have to pay anyway).
So you can only use it where there's a convenient wifi network. That provides you with an excellent excuse for visiting Wetherspoon's pubs (which all have free wifi) but many other places also offer free wifi, such as larger Tesco and Asda stores. (I've often used my netbook in the Costa coffee bars wihin Tesco stores).
If you get your internet at home through BT you can connect, free of charge, to the BT Hub of any other BT customer (and of their business customers as well) as long as they've not disabled that facility. (It's perfectly secure, as the 'home' and 'public' connections available through a BT Hub are completely separate; it's a condition of using the service that you leave it enabled on your own BT Hub).
If you've not got BT as your ISP you can still use their service but it costs you £6 per month. (That's what I do). You can also pay by the hour or by the day but it's far, far more expensive to do it that way.
There's no 'general' public free wifi service in the UK (except within the City of London); you need to use one of the services I've mentioned above (although the number of shops, banks, railway stations, etc offering free wifi is growing rapidly).
Some overseas cities have much better free wifi access. e.g. Paris has an excellent system (available across all parks and public buildings), which I've used several times.