Their colleagues and rivals (certainly for foreign language speaking staff) at MI5 the Security Service appear to have a more sensible website which at least offers SSL/TLS encrypted web contact forms.
SIS only offer a couple of snail mail PO Boxes:
"Members of the public who wish to contact SIS from the UK may do so via PO Box 1300, London SE1 1BD.
SIS has opened a special PO Box for job applications to the Service - PO Box 1301, London SE1 5UD."
which are pretty much an automatic betrayal of any people in foreign countries (British or foreign nationals) who might wish to offer their services to the UK Government.
The SIS website offers English, Spanish, Russian, French, Arabic and Chinese language versions of their website. The MI5 website only has English and Arabic versions.