I would agree with the other replies, 3G is useful if you have a need for it. My previous iPad 1 was the 64gb 3g model, and the 3g only got used a few times when on holiday, so I too was tempted to just go with the wifi version.
The wife and I are retired pensioners, also in the UK, we moved out in the sticks of Herefordshire when we retired, the thing is because we are retired we do tend to have quite a lot of holidays, usually 8 or 9 a year with quite a few short breaks too, so the 3G is useful for that, and for times when our main net connection goes down.
I just use a T-Mobile PAYG sim, which can also use the Orange network, it costs £2 for 24hrs £7 for 1 week or £15 per month, though they often have deals, at the moment you buy 2 days for £4 and get 1 day free. So you simply top it up as & when needed, so ideal for us.
AYK in the UK the iPad3 3G is £100 more, so if money is of some concern I would suggest you stick with the wifi version, but in my case, I figured as it's something you can't obviously add later, and it's just 1 tank of fuel in my car, or the price of a good evening & meal out with the wife, then even though I only use the 3G occasionally it's likely worth having again.
I don't agree that both the wifi and 3g have the same second hand value, if you look at SH ebay sales of iPads you will notice that 3G models are generally more expensive, though you likely won't recover the £100 premium when new. TBH £100 really isn't a lot of money these days, AIS it's 1 tank of fuel...lol, IMO if you think you will use the 3G then change it, or as suggested you can always tether to your phone.. so probably less important. Only you can decide, when I went into the Apple store in Birmingham on release day I too considered getting just the wifi version myself this time, but walked out with the 4G version for the above reasons.
Speeds on 3G in the UK as you know are not good at all, I get about 2 bars signal strength on T-Mobile at my home location, but if i switch to Orange it's full strength, but strangely I've not noticed any difference in speeds, both are still on slow side, at about 700kbps, so a big drop when compared to my 18mb net connection, but it works, and better than nothing for times when away from home, and of course with a PAYG sim, for us it does the job and is cheap.
Re. Tethering devices to the Ipad3. That is currently not possible, I've not seen any option for it anyway.. though may be wrong, as I often are..lol
Personally, if your current iPad3 is 100% faultless..ie no bad pixels, or small light leakage issues, and you can tether ok to your phone, I doubt I would be thinking of changing it now, especially as one would think the wifi version probably has better battery life, but it's your choice... it's that 2 weeks grace...lol it makes it too easy...lol