Much as I wouldn't want to encourage the loss of it, an 03something Pendolino from Glasgow to Euston (and corresponding EC service to Kings Cross), and a midnight-ish Pendolino to both, would probably be a broadly adequate substitute for the Lowlander. GW could do similar.
The Highlander would be a bit different, only really air would substitute for Aberdeen and Inverness, and the FW Sleeper, unless Megabus/Nat Ex stepped in with a new coach service, is the only way to do a weekend away to the West Highlands from London without a day off work - with day trains or air you'd arrive too late at FW/have to leave it too early, and it's too far to be sensible by car in one go.
That said, with regard to GW, the overnight service would never be *massive* in terms of loadings, so as long as the stock isn't knackered it might as well carry on rather than be swapped for an HST. It doesn't, unlike the Scottish ones, cost substantially more than running an HST, as there's no shunting or faffing about. Indeed, I must admit to being surprised they continued with the 57s, and didn't actually shove a power car on each end and run a Sleeper HST (with a motor-alternator set in each power car van area).
Indeed, thinking about it, the fact that the UK has one single big destination they all run to probably makes them more viable than Germany's web of destinations. The French ones might work were it not for the TGVs, and were it not for the fact that they threw away the business travel market (the only premium market there) by going couchette/seat only.