I get a bit fed up with the 2-for-1 deals so thought I'd share when I saw Specsavers are doing a "free standard varifocals" with any frame £75 or more.
My branch also do a free eye test if you buy glasses at £75 or over which they combine with the other offer. They deduct the £49 cost of the standard varifocal if you buy a better varifocal lens.
I have used the plain price on their glasses (no extras) in the past and been happy with their service, so you should have no problems if you just want a single pair of varifocals at a decent price.
Their nationwide offers are here on the web: http://www.specsavers.co.uk/offers
I have nothing to do with the firm other than buying specs from them!