In reply to Sam Edwards:
Is this for UK summer use, UK winter, alps summer, alps winter?
For UK summer use I would recommend a light synthetic ( primaloft gold perhaps ) jacket. Preferably one that stows in its own pocket and can be attached to a harness.
Personally I use the same jacket as a possible layer in alpine summer and UK winter but then have a beefier synthetic jacket ( Fitzroy is excellent, Rab Alpine Generator - if they still make them ).
If I could justify the money I would also buy a light down jacket for the alps, such as the Rab Microlight Alpine or a beefier jacket for alpine winter use.
So decide on your priority / intended use and what features you require. For example does it stow in own pocket, can it attach to harness, are the pockets where you want them and of the right size, does it pack small and easily, does it fit over all the layers you might be wearing, will it cope with the weather you will be in, is it light but still insulating enough, if hooded does the hood fit comfortably over your head or helmet if required. Try lots on to see what fits you personally. Remember, you will be wearing the jacket on top of other layers and possibly a harness so try them on with the other layers on underneath and think about harness access if required.
The decathlon down jacket looks awesome value but may not have the features you are looking for.