Currently the longest routes from LCY are ARN and FCO, both at approx 880mi and served with RJ70s. HEL would add another 250mi, so quite a stretch. The longest RJ100 route is MAD at 770mi.
Are any of the former Air Malta/Azzurra Air RJ70s still available (I know one has gone to the VAE as a VIP aircraft) ? They would problably be the only aircraft suited as they received additional at Air Malta's request tanks when built. I remeber they flew MLA-DUS regularly which is exactly the distance between HEL and LCY.