The adapters that you have, do they have an end like the top half of the picture of the ones I posted?
I am not 100% sure, whether the Mothercare Journey seat uses the same adapters as Maxi Cosi, or whether there are another set of adapters just for the Journey car seat.
Usually when you buy a set of car seat adapters, either as part of a travel system or standalone, they will be either for a specific seat/brand (e.g. Graco prams only take Graco car seats) OR you can buy ones which are sold as universal or called Maxi Cosi. This is simply because Maxi Cosi were one of the first brands to offer compatability with a large range of pushchairs. Over the years many other brands followed suit and made their own car seats compatible with these same Maxi Cosi adapters. It's a shorthand in the industry, basically. Then just to be confusing most pushchair manufacturers will have a car seat of their own which they would preferably like to sell you, rather than a competitor's so they are never especially clear about which exact car seats can be used with the pram.
So you will definitely have Mothercare adapters because one end of them will be unique to the Journey travel system, but what you need to work out is whether the other end of them is just for a Journey car seat, or whether the other end will take (several brands including) a Maxi Cosi car seat.
There's no isofix base available for the Journey seat, but if you have got or can get Maxi Cosi adapters for your pushchair, then you can put loads of possible car seats on it (about 35 at my last count) of which most of them, probably over 30 are compatible with an isofix base.