In the US, you might have more luck applying to applied mathematics programs since these often require less research experience and are more based on grades. I know a lot of string theorists are moving into math departments anyways--there's less pressure to bring in big grants.* Two schools outside of the top 10 I know have switched at least some of their QFT and string theory faculty to applied maths are UCSD and UC Davis (my current institution). A quick search will easily turn up more; a semi-accurate ranking for applied maths programs can be found at http://tinyurl.com/3au2bjo
, while for physics it's here: http://tinyurl.com/3jgepef
. Schools good at both are likely to have excellent mathematical physicists in the applied maths department, though there's certainly not 100% correlation.
If you apply to US programs, you will almost certainly have to sit the GRE (and either the Physics GRE or Maths GRE), which are non-trivial endeavors in Europe. The MGRE and PGRE are considered quite hard, and you should aim for a near-perfect score to get into top universities. Most applied maths departments will take either, so take a look at both and see which one suites you best.
*This also means its somewhat harder to get a graduate research stipend in applied math, but that's a problem in elementary theory in physics departments anyways. Regardless, you'll probably be a teaching assistant for at least 2 years, and probably longer, during your PhD. A bit of a drag, but not the end of the world.