What car seats to take to the UK? Two week trip - almost 2 year old and 4 year old

[Philly Mom]

In September we are going to the UK for two weeks. In the past, My FIL has always picked us up in his car and driven us back and forth from the airport. I just squeeze into the back seat for those two trips (about 2 hours on the motorway). We then take his car for the week and he drives their spare car. MIL does not drive more than a mile at any time and only for emergencies. FIL has a 5 year old BMW. It is an English BMW, fabric seats and the back seats have a curve to them. Last time when we went we took DD1 RF in a marathon and DD2 was in her keyfit. I was able to just squeeze into the middle seat with those two seats. It was not comfortable, and I had no seat belt, but we made it both directions. Now DD2 is RF in our marathon/boulevard and DD1 is FF in our Pioneer/Maestro. I can fit between the Maestro and Marathon in DH's sedan (VW Jetta) but it has a flatter seat than my FIL's BMW. While the trip is not until the fall, I may need to get seats in the next couple of weeks for a sitter to drive the kids so if I can pick two narrow seats now that I can also travel with, that would be great. Otherwise, I can only think of less than ideal solutions. Less than ideal solutions include: 1) FIL borrows a car from someone who has a wider back seat (may still not fit me with two car seats we already own); 2) DH takes the train into London and then takes a train out towards his parents and then FIL drives 20 mins+ to pick up DH at the nearest train station that goes into London after he drops us off; 3) we rent a car for our entire trip that we only need to get to and from Heathrow; 4) DD1 moves to a high back booster just for the trip (she is rather compliant but will probably sleep from the airport to my in law's house).

Ideally, I would find a more narrow convertible (width of the keyfit??) as a spare seat and then use Maestro because that seat is so light. I really think this will only be an issue this year and maybe next year. DD1 is 34 lbs and at least 38 inches now. DD2 is 23 lbs. She is pretty short. My kids are 20th percentile for height.

[Joolsplus2]

RideSafer Travel Vest for the 4 yo? For the trip, though...but it's not great for sleeping, but if you're right there to lean on, who cares? (I'll gloss over the no seatbelt for you in the middle seat, it sucks, but try not to crash, k? ) For the spare convertible you could do the Evenflo Tribute or Scenera Next http://www.walmart.com/ip/Cosco-Scen...Print/41126739 (not the regular scenera, that's a pain to install, this gets super reviews for installing RF)