OK, as far as fox mustang parts that you can use
-Engine and all front accessories are interchangable
-Trans (either AOD or T5 if you want to convert it to a 5-speed)
-If converting to a 5-speed, mustang clutch and brake pedals bolt right in
-Exhaust (H-pipe and tips just need to be lengthened a bit for the bird)
-Shocks/Struts are interchangable
-Springs (but get heavier aftermarket ones instead of stock mustang ones)
-Brakes (4-lug only. 5-lug conversion gets complicated, see below)
-Only car that has a bolt-in 8.8 rear diff is the 87/88 Cougar XR7. Even the V8 cougar LS and tbirds of those years only got the 7.5. The rear diff from a 94-04 is the right width as the fox body bird, and is an 8.8 instead of a 7.5, but you will have custom axles made, or convert to 5-lug wheels, but then you have to change your front spindles because the shaft is a different diameter, and then you run into issues because the front control arms for the bird are longer than for a mustang, and the ball joint has a different taper, so you can't use the 4-lug control arms in the 5-lug spindles, and the only control arms that are the right lenght and have the right taper are for the MarkVII, but they don't have a spring perch because they used air ride, so that leaves you either modifying the ball joints, or the spindle, or the control arm.
-If modifying the engine, get an A9P or A9L mustang ECM and the mass-air conversion harness, or you could just cut the MAF connector off any ford in the junkyard and extend the wires yourself, since those conversion harnesses are probably not so readily available to you.
Parts that definitely will not fit
-No body or interior panels interchange
-Driveshaft is a different length
-K-member is different, as are the motor mounts
-Fuel tank is different
-Fuel level sending unit is different, and within the fox birds they are different depending on whether it is an analog dash or digital, and both of them are nearly impossible to get in the US, so in the UK it would probably be impossible to get a replacement if it goes bad.
That's all I can think of off the top of my head.
-91 Cougar LS, coming soon, complete overhaul
with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
-"Your buddy Mike is INSANE!" -ClintD's dad