Good point Harry, but if the workers are employed in the UK, even if they work for a foreign company, will have PAYE and NI deducted. They will still spend a good amount on rent, food and entertainment so UK plc still benefits.
While it’s also true that all public tenders have to be advertised throughout the EU, I believe the awarding contractor can insist on certain sub-contracts being awarded to companies or suppliers of their choice. I think this is how France, Italy and Germany get round the rules. The Uk seems not to read the rule book!