The idea of whether or not two people can legally get together and kick the s#%t out of each other is an interesting one. I'm from the same country as the OP and in my experience, this is not the case, although it can be a grey area.
Hmm, well then. In the UK, a rather infamous case (the "Spanner Case") is sometimes used to demonstrate that the courts have ruled that consent in itself does not mean assault is lawful
Full ruling of R v Brown
<-- for the brave
A couple of key points that make this relevant: the charges were brought by the crown, that is, not because the "victims" had changed their mind about consent (they hadn't); and actually the ruling expressly mentions cutting the skinů which is at least curious, from a moral point of view.
Note that this judgement remains controversial, and if you do look at the full text (and search for "fighting" for example) you can see that the lords were aware of the potential implications of the judgement relating to things outside of the, um, particular activity that this case was originally about (i.e., people up to something that sounds to me even weirder than aikido). It's a handy case to cite in conversations in which even the merest whiff of a smokescreen of machismo honour fights is being thrown up to suggest you can consent to being beaten up. Basically, in the UK, you can't, and perhaps more usefully it shows that the law often isn't interested in what you want