New build. Obviously.
However the costs, time and risks associated with that option today must be more than one company is willing to carry alone. It might not be worth developing and financing a new locomotive design simply to support 3-5 years worth of HS2 construction contracts. The costs in converting older locomotives must be more attractive and offer a faster solution.
That might be different if we were talking about a replacement for the class 66 fleet. We aren't.
PS hopefully if they do build a new locomotive they undertake aural tests to ensure the engine note meets with spotterish approval.
An interesting perspective companies could merge up on a deal to get a new class of locomotive into the works. Since GBRF is in the title of this thread, we know they like to play alone and wouldn't want an outsider dictating any of their business decisions. Some of the smaller companies might have to do such a thing if the bigger companies don't hand down some of their examples. I mean, the DCR's of the UK (if any are left) don't have any class 56's to choose from anymore because they have been swallowed up.
What gets me, is why no non class 5 locomotives? Class 5's must be the most expensive to purchase. I can't even imagine what the last non class 5 locomotive new build was! It must have been a long long time ago. Perhaps a class 50? When I was a nipper double headed 25's pulled a hell of a lot of wagons. There is a Barnetby oil working that uses a 60 from Neville hill with about 4 oil containers on it, a 60? Really?
On your last sentence, I think the tone of the engine can be dictated by the exhaust system and I'm sure it can be modded to suit spotters needs. Not that its a priority mind. Whilst we move into the electrical age of things, we must realize that half the worlds population are used to loud engines and purchases have been made by certain folk because of that noise. A Harley Davidson, a Ford Mustang etc. With trains it also adds character and setting. I have 3 MP3's that use the engine sounds of a HST as part of its soundtrack. Look at the business Hornby/Bachmann do with DCC sounds of locomotives and how people choose their favourite engines in Train simulators? There is an economical reason in making products play a tune a certain way, and that is never going to change. If you take that away and make everything electric a certain amount of interest wanes.
Any loco sounding like a 20 or a 40 would get thumbs up from me.
edit - Tried to find out if the 68 was class 5, couldn't find confirmation. Classed as mixed traffic, so probably not. Apologies if I made an error. Seems to be high on HP and the price fits so maybe it is.