You should be very cautious since it sounds like a scam. You need a Brit solicitor (lawyer) to contact these people to see if they're serious. Do not send any advance payment.
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I agree this reads like a Nigerian scam. I would be very surprised if this was legitimate. If you believe it may be, because you know the wealthy uncle who died, you should contact and have the approval of your own solicitor in the UK before part with a penny.
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Sounds like a complete scam. There's NO reason that I can imagine that you'd need to send a cent before meeting with the attorney, so unless you can INDEPENDENTLY verify that these people are who they say they are, ignore 'em, or better yet, report 'em to the police.
So this lawyer is saying you have to retain him before meeting him? Really, what does that sound like to you? I suspect that this person has stolen the identity of a real British lawyer,and that the wire info that you are being given will not match up with the account of that firm. When you get to their office no one there will know anything about you. Call them and ask to speak to the person who supposedly called you- see what happens