Well,

My idea is that you enter the values as numbers only, like 60000 starting
in adjacent column you enter the currency, like £...

You would have a table off to the side of currencies and exchange rates

So assuming your first entry is in A2 and in B2 you have a currency of £

and assuming your table is in X2:Y5

Then make an entry in A3 and place corresponding currency in B3

In a new column say C2 enter =A2 (you can format this as £) in C3 you would have a formula like:

=C2-A3*VLOOKUP(B3,$X$2:$Y$5,2,0)