I did a quick comparison with Wise (previously TransferWise)
This is cheaper for GBP->EUR transactions over £800, but a lot slower (arrives "Friday" instead of within 5 hours. In practice I've found that Wise transfers are normally nearly instant). It's quite substantially cheaper for large transfers (e.g. about £2000 for transfering near their max) and possibly faster. I can't tell if you can lock in a rate like with Wise.