1) I need to obviosuly take the TestDaf exam which I am hoping to do in June this year (after completing a 4 week language and preparation course)... what I want to know is whether I can apply to Uni already without having yet done the exam ( and then just send them the certificate if/when I pass)...
Depends on uni, but usually you should be able to get a "conditional offer". Just call the uni in advance.
2) Secondly I have read that its on average about 500 euros per semester (term) ..... but is that for Germans or also for EU foreigners? And if it is just this little for German residents - whether having a German passport (as well as my British one) will help me in any way ( in terms of fees) ? NB: I have never lived in Germany but my dad is german and I have family there.
It's 500 for everyone, depending on how big your family is or where you want to study, you may not pay fees. Eg. Rheinland Pfalz and Hessen don't have fees, in Baden-Württemberg and Bayern you will be exempted if your parents have 3 or more children.
You are eligible for BaföG as a EU citizen anyways, so no, there are no big financiel advantages. Maybe if you plan to get a student loan, but I don't know for sure.
3) I know that with most Uni's (unlike the UK ucas system) you just apply directly.... but is there like a certain system for foreign students applying?
No, you apply like everyone else, the application sheet may be a little bit longer, though. The ministry of education plans to establish a central selection system in the near future.
4) does anyone know the general deadline for applying to German Uni's for this year (2011)?
Deadline for the summer semester usually is January 15th, winter semester is July 15th. If you want to study a course which doesn't require taking part in the application process you can simply go to your uni and register until the first week of the semester is over.
This information is what I know from my uni. In Germany many things vary from uni to uni so don't take it as 100% true.