Club Triumph

Hi folks,
I've been on here for a while as i have previously owned a couple of Heralds.
i've just bought a brown Toledo (was on ebay last week, you may have seen it?), has a full MOT but clutch was dodgy.
WE drove up from Portsmouth to Slough to collect it, with the intention of towing if we couldn't get the clutch to go with a bleed.
Thankfully i got a bit of a bite so drove home, the car working ok. The car wasn't really happy going above 50 as it started to misfire, but i did get it faster on occasion.
I wanted to see what the problem was as i know it should be absolutely fine at 60 plus.
Noticed that it had been fitted with a cheap electronic ignition (SimonBBC styley), but was still running through a ballast coil, but unballasted (judging by the volts going into the +). Fitted a spare 12v unballasted coil i had and no difference.
Plugs were black so i cleaned them off, air filter is pretty dirty so i ahve ordered a replacement K&N (to go in standard box).

Today i took it on teh motorway for my commute to work and it really struggled to go along at 50, so i came off at the next available junction and checked the plugs thinking it could be that. They were white, so very lean.

My question is this: Could it be that there is some crud in the fuel line that is causing the car to misfire at higher speeds/rpm, but clear enough to not noticeably effect it at low speeds? Plugs are blackish when driving around town this afternoon.
Thoughts are appreciated!

Have you thought about doing a compression test on each cylinder? Wonder if the valve seats are iffy on unleaded?

Have you stripped and cleaned out the carbs?, could well be some dirt in the carbs. Had similar issues in my GT6 with sediment, if a car has been laid up the inside of the tank can rust above the level of the fuel and small particles of rust can find their way to the carbs. My car even manages to get sediment in the carbs even with a filter before the fuel pump and I've now added another filter just before the cabs, so it's triple filtered, one before the fuel pump, filter within the fuel pump and filter just before the carbs.