I have a one-way ticket to London, I intend on buying another one-way to get back since it suits me better.

On a one-way ticket I cannot buy *A add-on coupons since they are valid only on r/t fares.

Does UA sell any European add-ons for one-way fares? Are there any other options worth considering (apart from Europebyair)? I cannot do a r/t since I need to start from LON, go to TLL and finish in AMS.