Access to voicemail while roaming requires a PIN for security reasons, if a PIN was not been set-up while in the UK before leaving - your a bit stuck, there is no default PIN.
Your best chance is if one of the Forum Team here offer to help and set-up a temporary PIN for your voicemail, but may take a couple of days for them to get round to your post.
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BB 200 - Hub 3, Phone Talk w/ends, Mobile Sim only. Customer since 1993.