So can I have the inbound (pop3) as a different ISP to the outbound (SMTP).
Yes. This is perfectly normal. Lots of people do it all day every day. The POP3 connection is the email you want to pick up, the SMTP connection is whatever you can get, which is typically the server owned by the ISP providing your current connection.
Also how do I set it up so that I can send emails from my laptop when I am travelling and have logged onto an anonymous ISP via WiFi.
Choices include:
(1) Find out the anonymous ISP's SMTP server and use that.
(2) Arrange with the ISP providing your email account that you can use their SMTP server even when you're not connecting via them; this will typically involve some sort of authentication, to avoid spamming.
(3) Use the web mail interface to your email account.