How did Dutch Marines strengthen Britain's NATO amphibious force?

Did Dutch participation supply a major part of the United Kingdom's amphibious hull capability?

No; the Dutch contribution was primarily a Marine formation that expanded 3 Commando Brigade into a heavier multinational brigade, not a replacement for British amphibious shipping.

The Royal Navy and Royal Marines provided the core three-unit 3 Commando Brigade. Dutch Marines could expand it toward a heavy brigade of roughly five battalion equivalents. That multinational experience is connected with the brigade's ability to absorb additional formations during the Falklands campaign. The northern-flank account describes the brigade facing a much larger Soviet force and an expectation of extremely heavy losses. The opposing formation is ultimately named as the 14th Army with naval infantry after earlier guesses are corrected. The Dutch contribution discussed here concerns troops, while British shipping still provided the amphibious lift.