I want to use the phone in the UK, when I'm travelling, so want the UK SIM to pop into the Canadian cell only when I'm in the UK. From posts on this forum I gathered that I would need to get any cell purchased in Canada unlocked for use on other networks. I hope I'm making sense — I'm a cell-phone newbie.
I recently did this exact same thing. I used a Sony Ericsson phone that was originally used on a Rogers contract. This particular phone was quad-band (and needs to be to work in the UK and Europe). It worked fine with my Speakout SIM card, because of the Rogers lock. I don't know which, if any SpeakOut-sold phones are quad band, so you'll have to check your phone out before you go any further. To prepare for my trip to England, I purchased an unlock code on-line for the Ericsson phone, and unlocked the phone. Cost less than $10, and worked fine.
When I went on my trip, I brought the Ericsson phone and used the Speakout SIM while I was still in Canada. When I arrived in the UK, I went to a CarPhone Warehouse, and bought 10 pounds in airtime, with a free SIM card, for Vodafone. This gave me a UK mobile phone number. I was able to use the phone to call friends and motels in the UK, and also to call and text back home. Everything worked like a charm.
If you are going to use T-Mobile/Orange, check to see if they have a special plan for calling/texting internationally. Vodafone did, and it was a big saving.
One other thought… If you find that your Speakout phone is not a quad-band model, there's always the option of buying a cheap phone there when you buy your SIM card. All of the mobile providers sell very basic models for £10 or less when you buy some airtime.
We didn't need to use our phone that much during the 3 weeks in England, so we still have a bit or time left on our SIM. We brought along a netbook, and used free public wi-fi for e-mails to keep in touch back home.
Enjoy your trip!