Public officials can be easily replaced against their will – the system is designed for that. A king, on the other hand, remains a king until they die or abdicate. When that happens, the throne passes on according to an established line of succession. Any other king installed by a conqueror would be an illegitimate pretender. The conqueror could in principle force the parliament to alter the line of succession. But because the UK is a common law state without a written constitution, it's far from obvious if the parliament can actually do that under such circumstances without consent from the monarch and their heirs.