There is no reason why any single body of unelected people should have the authority to bock parliamentary legislation

They do not have that authority. That is what the Parliament Acts are for. Though we may be fortunate that the hereditary peers (now gone) being a completely bonkers anachronism was a major reason for them.

Howard Jones put the situation very clearly above.

At a UK level the House of Commons is where democracy happens. No blocking second chamber, no president, the monarch can't have influence - and neither can any deity.