I don't know how to help re voltage, but I found when I went to the states and I was pumping, it was less stressful for me to take a handheld pump and rent a big one while I was there. I contacted a local lactation consultant and she rented me a symphony for two weeks. Also, Medela have offices in the UK and they can probably help you.
And this is just me....but rather than lugging a baby monitor all the way there, I found it easier for my mother to buy an inexpensive on and use it there. If you a travelling with a baby, it's a good idea to travel as lightly as possible. When I travelled with my kids when they were infants, I put a message out on Facebook that I was looking to borrow some baby stuff and people were jumping out of th woodwork with offers of stuff. You can buy a baby monitor at Target for like $20. Also, just another thought, I also bought an inexpensive car seat and I leave it at my parents house for my trips to the states, as UK car seats are illegal to use in the USA, it's just easier and worth the money. Good luck!