The survey does not ask for any personal details, but it does nag a bit, with most of the Questions being Required ones. The survey does promise anonymity and confidentiality, but MI5 are not the only people who might be interested in tracking or tracing the people who choose to fill in this survey.
The professionally paranoid amongst you may wish to use Tor or other proxy servers, someone else's WiFi access point etc. and to remove your cookies, fake your web browser user_agent string etc.and make sure that you do not use the SSL version of the website, if you want to keep your real IP address and some other details private (unless it is more important to you to hide your browsing details from your local systems administrators or ISPs, than it is from MI5).
There is a Captcha to reduce spam, but how this Government website survey complies with the Disability Discrimination Act 1995 is a mystery - perhaps they need to read the PAS78: 2006 Publicly Available Specification Guide to Good Practice in Commissioning Accessible Websites (.pdf) produced by the Equality and Human Rights Commission.
The MI5 website could really do with some Syndication feeds, (RSS 2.0 / Atom) like millions of blogs, including this one have, and like the FBI.gov website has had for several years, especially for the What's New section and the Terrorism Threat Alert Status.
They could also do with publishing a Mobile Phone SMS text and MMS picture / video clip receiving number.
Their SSL encrypted webform is ok, but it appears to be just a black hole into which information disappears without any acknowledgment or thanks - once bitten twice shy.
MI5 should also publish a PGP Public Encryption / Digital Signing key on their website, which has its uses both for securing and authenticating email , but also for postal correspondence with or without enclosed digital storage media.. This may not be UK Government Approved Cryptography, but it is the public key cryptographic system which most of their potential informants will have access to.
All of these methods of contact should be staffed with humans 24 hours a day, 7 days a week, and should have adequate technical infrastructure to be protect them from spam, viruses and denial of service attacks etc.
In terms of content, there are obvious limitations about how much information the website can publish without compromising national security, but since they are competing with the Secret Service MI6 and with the Metropolitan Police Counter-Terrorism Command for suitable staff, they really do need to make an effort with different levels of detail, for different audiences and age groups.
It would save us some hassle here at Spy Blog, if they made their entry requirements for potential job applicants in terms of age and nationality much clearer up front.
They also need to distinguish more clearly between the requirements for people wanting careers as MI5 intelligence officers or technical or support staff, and potential intelligence assets in the UK or overseas, or Covert Human Informants.