Need to show Currency

I have a large spreadsheet which I export from accounting software with multiple currencies.
The currencies show as follows:

€20,502.75
£886.49
$6,460.00

Obviously there are thousands more lines.

I need a formula which will convert these all into GBP.

Currently I am manually typing USD, GBP or EUR next to the line and then using =IF(J2="GBP","1","0"), =IF(J3="EUR","0.84","0"), =IF(J4="USD","0.77","0")
I then sum these cells and multiply the value so it comes back as GBP.

This is very long winded.

If anyone knows a formula which can take me straight from $500 to the GBP value for mutli currencies that would be great.
If not, then just a formula from £500 to show GBP in the next cell.

Thanks in advance,
Matt