|The first thing I would do is verify the problem.|
If you have £150 in A1, I would use =ISNUMBER(A1) in some other cell. If the result was FALSE, then the contents of A1 is text, regardless of how it was formatted.
One possible way to convert the contents from text to numbers would be to select all the values and use Edit Replace. Put a £ in the Find what: field, leave the Replace with: field blank and click Replace All.
Hopefully your ISNUMBER test is now TRUE.
If not, select any empty cell, press Ctrl-C for Copy, select your list of values that no longer have the £ and do an Edit...PasteSpecial...Add.
Now format the cells as currency with the £ if desired.