Is it advisable to mix different compaines products, say a laney head with a warwick cab? And for £500 would a combo be a better choice than a head and cab?
Depends what combo, what head and cab, but yes it's perfectly advisable, just make sure you use the right resistance cab with the correct head....OR BOOM!!
Epiphone Dot-335
Fender USA Tele
'82 30th anniversary Les Paul goldtop
1965 pre-CBS Fender Jaguar

turbo rat
+many more

Fender Twin Reverb
King's Full-tube 100 watt half stack. - £300