I've never seen trim issues on the larger B737/B767/A319/320/321 series, and I can't ever recall seat shifting on Domestics either, there's no curtain issues there either. This includes very low loaded services, e.g. on Saturday afternoons. The risk is on LCY's Embraer service, where I've certainly seen it happen, though Golds and above are sometimes exempt. On the 12:35 to LHR I can't see any risk of being shifted.