If cell H5 contains X return value from H5, else return value from cell ##

I am stumped on something i thought was very simple to acheive;

If the cell contains a number or better still starts with lets say letter "P", then return the value from that cell, if not return a different argument.

I am trying to override a calculated figure, if a manual figure is entered; The manual figure will ALWAYS take precedent over the calculated figure.

I have tried the following scenarios, all of which do not work entirely;

=IF(LEFT(H5,1) = "£", H5, SUM(AO5:AU5))

I appreciate that it is only looking for the £ sign, i have tried various combinations

=IF(ISNUMBER(SEARCH(,H7,1)),H7,SUM(AO7:AU7))

This works, but returns anything entered in cell H7 not just if it contains a number

=IF(H8>=1,H8,SUM(AO8:AU8))

This works, but returns anything entered in cell H8 not just if it contains a number

Again i appreciate why they return anything, i just don't know how to write the argument correctly.

Many thanks