You will likely have problems trying to take Kamagra as its unlicenced and non official, knock off medications in the UK.
But going to Asda and getting a private prescription of Sildenafil you should very well be fine. And if not fine, all you risk is them being confiscated. Ask the pharmacist to give you a receipt for the prescription to show if they ask you and tell them its prescribed for your blood pressure - which is kind of true and one (very unlikely but possible) reason it can be prescribed.
Or as an off the wall idea, why not find out the address of the hotel you are staying at in Hungary, and then mail them to yourself a day or so before you set off marked Private and Confidential. Room 147, Joe Bloggs, The Hungary Inn.
They will likely keep it to one side knowing you are due to turn up if it arrives before you do. Even drop them an email asking to do this too.