Routeing MAN - BRS you would have to use the Monty SID. There is no way of getting you from UN862 into Bristol.
Also the standard routeing in Cardiff/Bristol from the North is to route via Talga and BCN which is handily on N864!
Easy deps from Liverpool going onto LEXX/LPXX use the Monty departure but the vast majority of Manch deps go via Nokin.
Hope this helps
CF