Thanks for suggestions.
I've got some clip on mudguards (yes, bikes come with mudguard eyeylets, but not with mudguards)
Two spare tubes
Wrench and allen keys of appropriate sizes for basic stuff
Reflective vest (plus couple of those reflective stips that you put around your legs)
Some decathlon energy gels (Will add some protein bars).
Long sleaved base layer, and padded cycling tights
Not yet completely sure what I'm going to bring food wise. I was planing on bringing some peanut butter sandwiches (maybe something with some dairy based spread and some meat), and a banana or two.
I've not yet fitted watter battle cage, but I have Decathlon near my place, so I'm just gonna add one, as carrying water in backpack is not that convenient :D
Any suggestions are welcome...