@TheDoctrineOfSciAndNatureClub (and all responders): Thank you for your helpful comments.
In our company, the bonus is specifically awarded by our employer for our achievements and loyalty over the respective whole year (as we don't get any other form of extras or benefits nor any contribution to maternity pay even if we've stayed with the company for over 7 years), and this is why I've come to think that the relevant proportion of it, for whatever period is being considered, should be included (as the current rules generally do allow bonus payment to be considered after all). I understand that in many other cases extra payments might be more discrete though and so it becomes more difficult to handle by HMRC, as you say.
I agree with you that the cases where self-employed women have a choice to pay themselves a good salary or even a bonus are probably rare, as most are likely to be struggling to survive. The fact that it can and has been done before though, shows that where there is a loophole and people can make best use of it, they will; though only if they know, or get advice on how to go about it. In the hope that I might get some ideas for my particular situation, I asked my question here. I am usually terribly useless and non-clever about opportunities that could lead to personal financial gain, and only because I will be struggling badly while on maternity leave (having to leave my DH behind to live with my family outside UK for half a year as we won?t be able to afford the rent during that time), I thought it would be a good moment to start getting a bit more savvy about financial details.
You're right, the system works best for those who can plan ahead and influence proceedings. Since many women are not in this position, however, it doesn't seem a fair basis to count and rely on. Had I, for instance, known about the bonus rule AND my pregnancy a few weeks earlier, there would be no problem as I am sure my employer would have been happy to pay my bonus in January rather than November. My view is that the law shouldn?t be so reliant on chance, good luck, 'cleverness' and position of power.
Just a note about the 'tone' comment: I am indeed very grateful for the contributions relating to legal information that were made by the responders. My original question/query was about getting advice regarding bonus regulations or other ideas with respect to what I might be legally entitled to - like planning ahead, only with very little time left as I am in the middle of my qualifying period now!
The responses, however, somewhat shifted the focus to interpretations of fairness, ranging from 'these are the rules and you won't change them' to 'the rules are fair' and 'the system is logical'. I objectively stated my view on these interpretations as I do not agree with them, and used an example to make this clearer. If I sound frustrated in the very last part of my last sentence, then this is due to being left feeling like everybody agrees with the current system, with only me being weird for finding the rules unfair, while my actual call for help has disappeared into the background. I apologise if anyone felt offended, this was not my intention.
I totally agree that this fairness-of-SMP debate would be worth a separate thread rather than moving this thread away from the original question. For the moment, however, my main and urgent interest remains in receiving any advice available regarding the question 'what can I do now in my situation that isn't illegal or unfair'. If anyone can think of anything...
Thanks to all for reading so far and for making an effort to contribute to the discussion.