What you can do is change the "log in" link on your web site to the page you want users to go to in 365. In other words if you want them to bypass the portal page and go straight to a team site, change the link to <domain>.com/teamsite (in a P account). They'll get the usual log in page but then get taken straight to the page you designate in the link.
As a back up, you can also add the redirect to your 365 public site home page. A meta refresh tag doesn't have to go in the head section. Use this generator and put it anywhere on the page and it will work.
Make sure the meta refresh is formatted like this though:
<meta HTTP-EQUIV="REFRESH" content="0;url='http://choiceandcontrol.co.uk'/">