Actually pharmacies in my country have begun to sell Valtrex over-the-counter, so it seems to indicate a small dosage like 500 mg would be unlikely to do much harm. On the Lerner protocol dosage, you would need to have liver values monitored. Anyway, I don't want to give any medical advice here, but as I said, you are unlikely to find any doctor in UK monitoring antiviral use.
There are some other things that you may try first if your main goal is to treat EBV. Vitamin C (liposomal or preferably IV) have been shown to decrease EBV antibody titers (
source), which is really interesting. Monolaurin has quite a bit of anecdotal evidence. Andrographis is a third one that is commonly used.