2) I was questioned about the starting date during the interview. While the panel chair said it would be flexible, then the HR rep in the panel said that the position should be started within 6 months since the advertisement of the postion (in my case January 2012), which she said was the UK legal requirement or something. My question is: is this the case when it comes to the starting date for accepted jobs? I was informally told by staff members during lunch that it was flexible too. I imagine this would be the most important factor that I care at this moment. Ideally I would like to defer the starting date to next summer or even later, for family as well as academic reasons.
4) Final question, was the decision mostly made by the department chair? I felt that he liked me the most among the panel. This question is mostly out of curiosity...
Re: 2), you are not UK/EU, right? Then special rules apply. (Welcome!) The univ will have to request a CoS (Certificate of Sponsorship) for you from the UKBA (UK Border Agency). The scenario is that the CoS cannot be requested any longer than 6 months from the time the advert went out, and the CoS must then be taken up within 3 months of its delivery. That gives 9 months between advert and your absolutely latest arrival at the univ. There is, additionally, a nation-wide cap on the number of CoSes in any month, so there might be an extra month in there, max. I've not yet known the UKBA to be particularly relaxed about their rules, so your might have a good hard think about this timing. If you delay, the UKBA will not issue a CoS and the whole search will need to be run again (or they will simply go with whomever was second to you, if that person is UK/EU or can start sooner). If you rock up at Heathrow with an expired CoS, they can and will throw you back out. In this case, heed the HR person...these rules are relatively new and the staff and academic ctte members might not have heard of them.
4) Depends on the univ but there will have been a committee, and everyone should have gotten one vote. The dept chair might be able to turn opinions, but not necessarily. I wouldn't think about this again, if I were you. It's probably not healthy.
Congratulations.