hiya i'm doing joint honours spanish and portuguese. I did portuguese from begininers and i've jsut done my first year at uni. To be honest you will find it easier to do portugese from beginners rather than spanish ebcause as the majortity of people doing the subject are ebginners and the class sizes will be small it will be easier o learn. At my uni the beginners spanish have to do literature and history modules that post a-level studetns have done and it is really tough for them. They also have to go into language classes with the post a-lvel students in the 2nd year so again it is quite tough.
I wouldn't reccoment doing single honours for lang ebcause you will find yourself ahving to do loads of modules in random things you might not like to get enough credit points. Then again if you lie a bit of variety you might enjoy spening half your time doing random things like anthropology. However, if you do single honours french you might be able to do beginners language modules in spanish as an open unit but you probably wouldn't be abel to learn about the literature adn history adn you probably can't get joint honours however many creidts you do in the.anish lang options.
If you think you'll enjoy portuguese then go for it I've really enjoyed it. However, it is true that it is less useful than spanish but if you don't want to go into a job directly related to languages it won't make any difference what so ever what languages you do... (a big exception would be teaching i've recently found out that because i've not done french i can't get onto a training course and my portugese is useless in schools).
By teh way i'm sure there are more places where you can do spanish form beginners
Hope some of that was useful. good luck Em