Voting plans

So I have applied for a proxy vote (first time I've done that) because I'm unexpectedly going to be away on European election day. This means I need to inform my proxy of my voting decision. I am at this point genuinely undecided between Green and Lib Dem, so I'm open to persuasion.

( political priorities )

So I think mostly I want advice on tactical voting here. If it matters, I am voting in the East region. But if someone wants to point me to a really critical policy difference between the LDs and the Greens, that could be persuasive while I'm on the fence.