I think with a little work Hornby's Mk4 could still be pretty acceptable. I've rewheeled my Intercity ones, and plan on adding the resin parts that Keen offer and some flush glazing. Once that's done I think they'll hold their own. The main problem is the Class 91 and DVT.
In terms of the Mk3, I've stuck with Lima ones. The Intercity colour scheme is more accurate rather than using that horrible beige, and I think they're generally a slightly more accurate model without door entry lights and other more modern details. Plus as most of my Mk3s are meant for HSTs, it means at least all the coaches match rather than having a Lima tooling TGS and a Hornby tooling TSO, TRFB and TFO. Seeing a true Mk3a would be nice though, along with an RFM and BFO.