Well if you have access to BTFON because you're a BT Broadband customer, then you may not need the 3G side of things. There are also lots of places that have free wifi, such as some pubs, some fast food places, and coffee shops. Many of the hotels do charge for wireless internet access, but often you will find it's free in the bar area (or if you happen to get a room right above the bar!).
The 3G is a signal from the mobile phone masts, it's essentially just a faster connection - as we go further from the masts (say in rural areas), or in congested areas, the signal drops to a different type which although slower than 3G, still give good phone calls (on mobiles) and a more sluggish internet access (on mobiles and tablets).
Obviously the advantage of having a '3G' one is that you don't have to worry about finding somewhere with internet access. The downside is though that the battery will go down quicker, the tablet will cost more in the first place, and you will have to pay for using the 3G data.
For the cost of the data, you'll need to budget between £6 and £15 per month depending on which mobile company you use and the type (Pay as You Go, Pay Monthly, or 12-Month Contract). If you are unsure of the amount of data you will use, to start with just opt for 500Mb (500 Megabyte) or 1Gb (1 Gigabyte) - that will probably be enough for normal web browsing, as long as you are not watching videos on it, downloading music, or doing anything that involves heaps of images. A number of the "pay as you go" 3G cards expire the data allowance after a month (so you'll just lost it).
Naturally, when you have a WiFi signal, the 3G of it is not being used and it won't be eating into the 3G data allowance that you would have.
If the 3G side of things sounds complicated with and all these data plans and costs, then a wifi-only tablet may be the way to go for a simpler and cheaper life. It also gives you the excuse to treat yourself to a coffee to get the wifi access. You can get two or three coffees a month for the cost of a 3G connection.
We have just started using the
Google Nexus 7 here - we've only had it a couple of days, but it seems great. We opted for the 3G version (and bought a '12Gb over 12 Months' SIM from three.co.uk for £72 or thereabouts). The only reason we went for the 3G version is that we run RootsChat so it's jolly handy to be able to be connected all the time.
Trystan
