If GB is not at war with Spain, I don't think you can just send British troops to help the Portuguese. You can give them depots and supply, but not troops.
If you want to fight on their side you can declare war on Spain in the next diplomatic phase, but it costs the usual PPs. Or, if you checked Portugal on the "Minors to support" list, you can enter the war on their side immediately (but it still costs the PPs).
However, none of that should make corps disappear. As I understand it, you should be able to land British troops there, they just can't participate in combat if you're not at war. And of course the Portuguese corps should be able to fight as normal.
There have been a couple of other instances of odd behavior by the game where there are units of different powers, supposed to be treated differently, in the same place at the same time, so I suspect this is something like that. For instance, two powers at war currently can't have combat in the territory of a third-party neutral. And, even more like your situation, sometimes where there are three powers involved in a naval situation (like the fall of a blockaded port), fleet counters will mysteriously disappear.
I think these problems are getting fixed in the patch, and perhaps the fix will cover your situation too; in which case, you might be able to restore from the savefile. You still wouldn't be able to use the British troops in combat without a declaration of war, but you might still have the counters on the map.