Hi Noah_123,
You are right, in Box no 8 should be “Total value of all supplies of goods and related costs, excluding any VAT, to other EC Member States” - the amount your eu customer paid to you without VAT.
This amount (without VAT) should also be included in box 6 where the total of all your sales (UK and EU) should show up. Box 8 is only for sales to other EU/EC Countries.
Did you set up these invoices for foreign customers described in this article Invoicing in foreign currencies and also in this article VAT Returns Guide?
If you come to this point “VAT on EC sales and acquisitions” 
I would choose no 3 “No Special Treatment”. Have you tried this? I don’t have EU costumers but this is what I would click.
Is it not working, you can also enter any adjustments manually (described in the VAT Return Guide article),but do it only with care and after speaking to your accountant.
Maybe it is a little bit helpful.
Please note, I am not an accountant and I don’t work for quickfile.