If I were you, I would follow the advice from SW Herts consortium and register before their closing deadline of 27th June.
- SW Herts test registration form will be available on their website- http://www.swhertsschools.org.uk/ on or after 13th May, you are also able to register online. The benefit of registering online is that you get e-mail notification of test result.
- QE Barnet form will be available from the school website - http://www.qebarnet.co.uk/admissions_page on or after 1st May (deadline 23rd July 2014 noon)
I have experience with the above two only, but I guess other Herts school forms will be available from their respective websites.
I heard Bucks have a common 11+ exam organised by a consortium or council. I am not sure though and recommend you check Bucks council website or call their admission office.