I've done extensive work on the sound system of my Z4 and would like to offer some suggestions that I believe are not too expensive. I will forewarn you that it is possible that you might upgrade your speakers and then determine you want to upgrade the amplifier. This is not a given, but a heads up.
Good news is in Europe there are plenty of replacement choices, compared to the USA.
Footwell speakers are considered "mid-bass". There may be straight replacements available in Europe. I chose Hertz 165.1 speakers. Posts #54 and #55 cover that in
my build thread. Ignore anything I say about rerunning center channel wiring, since you don't have that setup. Footwell speakers are an optional change. I would not start here, if you are cost conscious. Start with the midrange speakers.
Mid-range speakers. These are 4" mid-ranges. Change them both in the door cards and in the rear shoulder locations. Look on UK amazon and buy speakers for the BMW E90 (more common) that are 4" - lots of brands work, like Rainbow, et cetera. Bavsound has a
Youtube video for doing the replacement that will help you.
I would then suggest that you wire in some tweeters up front, to the mid-ranges. These will come with built-in crossover. You will need to decide where you want to mount them. I would suggest doing these on the front mid-ranges. To give you an idea how to do this,
check out my post on this in my build thread. Please note, I did these for the rear 4" mid-range speakers, but you can apply the same technique to the fronts. You don't need to do the rears - the fronts are what matters most. Wire tweeters with crossovers just like I did, but in front.
That's my advise. Maybe others have suggestions. Check the UK z4 forum as there are others with your configuration and they may suggest speaker purchases as well.
Filippo