Generally the VAT you do pay to suppliers is deductable from the VAT you get. Problems are cross country because getting that back requires paperwork. And time.
Which is why it is standard in B2B transactions to NOT pay the VAT on the country of origin - which is called "reverse charge". They get your VAT ID, they check it against the EU database and validate it, they send you the good without charging you VAT. Obviously you still collect VAT from your sales, and as you have no VAT to deduct, it all goes into your local VAT payment.
In your above example you would get the sofa for 800€ (net price, no VAT added) and sell it for 1000 pounds net. The difference is your profit. VAT is passed on to the UK tax authorities. Note that there is no sales VAT you pay in Portugal.
WITHOUT reverse charge:
- You pay €984
- The portugese tax authorities, upon export, owe you €184 VAT you paid, which you will have to reclaim.
- You sell for for 1000 pounds, the collected VAT goes to the UK tax authorities.
Now, getting back the prepaid VAT is your problem. This is not double taxation as you have a legal right to get it back, but... for €184 it is NOT worth it. And for anything sensible in size - there is no reason to pay it in the first place, thanks to reverse charge.
If you insist on reclaiming, then https://europa.eu/youreurope/business/taxation/vat/vat-refunds/index_en.htm is the page you need to help you with the reclaim process.