That can be quite challenging. I use a spreadsheet (self designed) which provides 3 tables from which to select:
Table 1 = amounts paid in € remainder £ converted on ECB rate
Table 2 = amounts due in sterling converted to final ECB rate
Table 3 = Table 2 annualised ECB rate.
The ForEx average is the sum of all £ amounts converted during the year and is mainly just a check-sum against the basic average for Jan - Dec finals (and thus not used - just for information).
The only issue that arises is what to do with remaining sterling which is spent in sterling in the UK and not converted to €. This is where the tables take account of it.