Good luck on your move! We moved to London several months ago.
Regarding London Relocation ("LR"), we had a very mixed experience with the company. Our home requirements were not difficult to meet - a high budget, reasonable space expectations and a fairly wide geographic target within London. Our initial calls were positive. The agent asked us specific questions about what we wanted, where, etc. LR agreed we would have no trouble finding the right place. Our viewing day was great - we were met by the agent and a driver. We had a very busy day viewing 21 properties. We found the place that felt 'right' and proceeded with deposit, background checks, etc. The agent was great.
At that point, we were turned over to a contracts person within LR and the whole experience fell apart. The negotiation between the leasing company and the landlord hit a snag (not related to London Relocation or us). We were understandably nervous - our belongings were on a ship heading to London, we were 'on hold' with 2 small children waiting for everything to be finalized - and repeated calls to LR would go unanswered or returned days later. We were paying a large fee out of pocket to be guided through the process (~$6000) so thought we would be given a bit more attention. The lease was completed much later than planned. On the day we were to receive the keys from the landlord, London Relocation made a big deal about being there to help us. Then, the LR agent no-showed ... my husband made quite a few calls but never heard from the agent or the contracts person. The landlord and my husband waited awhile but finally proceeded without him. Several days later, the contracts person sent an email saying he had forgotten to notify my husband that the agent would not be there. (I think a phone call was in order) The email was not apologetic. Since then, we have not heard a 'peep' from LR. Nothing ... Their promotional materials claim they will provide help with accounts, moving, etc. We did not receive any additional help.
So, I cant say our experience was terrible because we did end up in a home we are happy with -- but if you want someone that will make you feel warm and fuzzy, LR is not the company. When I read reviews of other relocation services on this board, I realize other families have had much better experiences with their relocation services.