Some interestingly varied comments here - one crucial thing that must be remembered - there isn't much difference between any fingerspinner, unless one is wayward and one very accurate, on unhelpful wickets.
I'd prefer not have Giles, Harbhajan, Saqlain or Tufnell in any of them.
The only way you can really judge the ability of a fingerspinner is their bowling in helpful conditions - otherwise they really are all as bad as each other.
Dawson and Batty have both played Tests and failed in helpful conditions (SCG, and both have played in the subcontinent).
I would say Giles has played on a helpful pitch in 9 of his Tests - and he's done well in all but 2 of them (Galle and Kandy 2000\01). It's important to remember, also, that not EVERY subcontinent wicket will be like the stereotype.
IMO Giles, and Croft, are just slightly inferior to the like of Saqlain and Harbhajan, because they don't bowl doosras. Neither did Tufnell, he was also amongst roughly the same group IMO, and so was Peter Such. Almost a totally forgotten bowler, but a very fine one in the right conditions.
I don't really see what this obsession with Tufnell being superior to all other English spin is - Tufnell was every bit as poor in typical English conditions as the next fingerspinner.
So basically I think England's worst spinner who has played international cricket is Dawson and Batty tied - they're both as poor as each other.
Min Patel is another who I think could be fairly effective given the right conditions, but he should never have played when he did.
Oh, and Marcus Trescothick has never bowled spin in a Test-match, I can assure all of that. Maybe he might have done on some not-unusual instance of 500 for 6 at Taunton on a hazy August day as another meaningless match petered out into a meaningless draw pre-1999.
And to those who say Giles has never produced match-winning spells - first, remember that that term is a wholly unreliable one, but also remember that Giles has made a contribution, a big one, to the winning of one of the most memorable English victories ever - at Karachi in 2000\01.
Anyway, matches won ratios don't say anything about a player's ability - his record does that.