I'm another who was on the first GE 312 railtour (HRT's Grand Slam). It covered pretty much the entire GE electric network as far as Ipswich, other than Southminster and Braintree. It showed how successful EMU tours can be as it was so popular they decided to run another, I think 2 weeks later.
There were quite a few with the SR Slammers before they were withdrawn. The biggest was the Southern Belle, which involved all 3 SR TOCs (SET and SC provided the stock, SWT provided crews for running over their routes), and ran from Ramsgate to Weymouth. It used a CIG (1866) a VEP and a CEP, being a very rare 12 car working to Weymouth (needed special autherisation and monitoring due to the power draw). I did that, travelling in 1866's motor coach.
A big later was the Sussex Slammer, which was just Southern lines, using 3 of their units, 2 CIGs and a VEP. It covered a lot of Southern's Slammer routes such as Brighton, Bognor Regis (or Littlehampton, one or the other) and East Grinstead. That one didn't get pemission to run 12 cars to East Grinstead, so had to split into a 4 and an 8 for that part of the run. Did that too, on the VEP.
SET earlier ran a tour to commemorate withdrawing the CEPs, although they later got some back. I think there were a few other Slammer tours towards the end, although some were staff only.
c2c ran a small tour for their last 312 working. It went from Fenchurch Street to Southend via Upminster and Ockendon, back I think via Dagenham and Stratford, then onto Shoeburyness via Basildon. That was another one I did.
To extend to Multiple Units in general (which are just as rare on railtours as EMUs), Southern had "The Last Thump" with 3 205s just before they were withdrawn, and FGW ran a tour using 3 of their 101s a few days before they were withdrawn.