kevster,
I was looking into it and i think 3 charge £5 per month to add tethering as an extra “upgrade”.
It’s the only one I’ve seen. The rest either allow it, or don’t -no upgrade option.
It looks like most of the main networks allow limited tethering on 24month contracts. vodafone, o2, etc.
The secondary networks, those that are piggybacking the signal like the supermarkets, don’t generally even on contract. It’s the main thing putting me off Virgin, ‘cos the rest of their allowances look great value.
The exception looks like GiffGaff’s 1Gb “gigabag” – , monthly rolling contract, no long commitment, £10, tethering allowed.