As Dave says above, if you already have Halfords coolant in the bongo then you are fine to add a pre mix (or even neat) coolant of the same type in the bongo Umbongocat... IF however! (please pay close attention to this) You bought the bongo with the coolant it has in already,regardless of the colour, then it is NOT adviseable to add a euro spec coolant even if the colours DO match i'm afraid.
A very quick reason for the above is that the japanese (the bongo's domestic market) uses different colours of coolants for different types than we use here in the UK. You could unwittingly adding an incompatible coolant to the system, which can cause all manner of problems, including the coolant sludging up, and the chemical attributes of the different coolants could possibly react against each other. Most of the bongo's in the UK with overheating or coolant related issues problems stem originally from an incompatible coolant being added at some stage as a top up when they come over here to the UK.
If in any doubt of possibly mis-matching, i'd personally risk adding just plain water to the system, but strictly on a very temporary basis! You run the risk of the system freezing that way (as the anti freeze has been diluted) But i'd say that's a better bet than risking incompatibility.