United will have to make a good offer for Mbeumo if they want to attract him to the team. Because Tottenham has the advantages you mentioned, and it all depends on the best negotiation for all parties. Brentford will certainly want to get the benefit of selling Mbeumo. If price is not an issue for both teams, I think Mbeumo will consider Tottenham for the Champions League. What will United offer? Maybe a bigger salary, or they have other ways to get him.
They have discussed the offer with Brentford and Mbeumo himself but there are issues that the player may still be considering. Tottenham could guarantee Mbeumo playing in the Champions League but I believe in the long term he would be much better off considering joining Manchester United. It depends on the negotiations and the offers made by interested clubs to Brentford's demands and if one of those can be met then everything will work out well. Manchester United are probably a little ahead of Tottenham but anything is still possible.