A friend of mine would like to know if anybody knows of any Letting agencys in the Wirral that allow letting property out without the mortgage company knowing?

I'd think not,no. I believe it's a legal requirement to ask the mortgage company if you can lease out the house. If you didn't, and then something happened while there are tenants in the house, it's also likely the insurance company would use it as a loophole to get out of paying up for damage etc.
The only way to get around this would be to lease out the house undercover so to speak without using a letting agency, and then the homeowner is opening themselves up to potential problems if the tenant doesn't pay and you want to evict them.
Out of curiosity, why doesn't your friend want to notify the mortgage company?