Chester-Wrexham-Shrewsbury had jointed track until very recently, I can't remember if it's all been replaced yet. (The North-South Express is loco-hauled, and takes this route, though it may or may not meet your definition of fast running.)
Last time I worked over it there was still a small section on the Up line (northbound) a few miles south of Saltney Junction. The speed limit is only 60 there though (the recently laid Down line is 90 there).
Settle Junction to Carnforth is probably the best surviving stretch - assuming it hasn’t gone in the last couple of years. Nice scenery, 60 mph, every train I think will have opening windows (unless 158s ever do that route?), and for those who want the full works there’s even the possibility of a bus-seated Pacer doing the honours.
Machynlleth to Pwllheli has plenty too, although not particularly fast.
There’s still some sections round the Cumbrian Coast, although don’t think this tops 60 mph.
IIRC, the Cambrian doesn't have anything over 55 (or since thanks to ERTMS it's in KMH, 90). Cumbrian coast is 60 max I believe, though I haven't looked at that particular sectional appendix in years.
There's a bit on Anglesey near Valley that is jointed track and has a 75 limit I believe. York - Scarborough used to have some high speed jointed track too but I don't know if that is still the case - I have a memory of it being as high as 90 for Sprinters but this is going back about 10 years or more, and of course Sprinters don't currently work that line anyway.
Personally, I enjoy going down the Heart of Wales line. Virtually in its entirety it's jointed track, Only 60mph max (and only then in limited sections) but with the movement and the noise the joints create it feels like so much more.....