Was the Washington Naval Treaty a sign that the UK considered the US just as powerful as them

[OSR stands with Israel]

Keep in mind in 1899 the UK created a policy of having a Navy at least eqaul to the combined strength of the two next most powerful navies in the world but just 23 years later they agreed to the Washington Naval Treaty which limited the size of the Royal Navy to the US Navy.

Was this basically an acknowledgment that the US was now on par with them and if they went into a naval arms race with the US they would lose. 

[Orser67]

I'm more familiar with the U.S. side of negotiations, but my understanding is that the answer to this question is "pretty much, yeah". After WW1, the British a)simply couldn't afford to keep up the naval dominance they had had in the late 19th century, and b)had to recognize the enormous scale of U.S. economic power, and their ability to apply it militarily if necessary. Other European countries were also eager to avoid big military spending in the aftermath of WW1 (as well as a recession at the beginning of the decade). Meanwhile, the U.S. was looking to ramp down military spending as it moved away from Wilson's interventionist policies. So the treaty made sense for all sides, except for the Japanese, who felt the 5:5:3 ratio of ship tonnage (5 for the U.S., 5 for the UK, 3 for Japan) was unfair to their interests.

[Statilius the Epicurean]

More than that, it was Britain outright acquiescing to US global hegemony.

At the time Britain was trying for a bilateral conference with the United States to discuss the postwar order, and the nine-power conference called by America implicitly reduced them to junior partners who would be consulted alongside Japan, France and Italy. Churchill didn't want Lloyd George to send a delegation because of this perceived insult, but Britain was in an exposed position economically and strategically and had no choice.

The idea behind disarmament from the US point of view was that it would undergird a peace that would allow American capital to flow through the world unhindered, and prevent any power from using militarist means to create a territorial bloc that could be a counterweight to US economic strength.

For Britain it meant abandoning the two-power standard and a Royal Navy that couldn't defend its home waters and the Empire in the Med and the Pacific at the same time. So it was acknowledgement that the survival of the Empire depended on alignment with and the good graces of the United States if they wanted to be able to defend against challenges by regional powers like Japan or France.

[The Mikado]

Adding to a lot of correct points above, the Washington Naval Conference was necessary for financial reasons to prevent the US and UK from bankrupting themselves in a totally pointless naval arms race, especially from the British side. No naval force limit from the treaty would mean the US and UK would be pitted in a completely pointless neverending contest to build more battleships that the UK especially could 100% not afford.

Orser67 is right that the Japanese saw a 60% force limit as a grievous insult, mainly because they like everyone else worshipped at the altar of Alfred Thayer Mahan and found Mahan's claim that a navy 70% the size of the largest in the world is necessary to prevent aggression against yourself an important rationale for wanting 10-10-7 rather than 5-5-3, and did not find both America's and Britain's claims that they both had two oceans while Japan only had one convincing, especially as Japan had just shifted from British ally to potential adversary with the end of the Anglo-Japanese Alliance.

The Japanese did not realize that the sheer industrial capacity of the USA meant allowing Japan 60% the size of the US Navy was actually a concession, not an insult. As the Japanese would learn the hard way two decades later, America had the financial and industrial wherewithal to support a fleet FAR larger than 5/3rds the Japanese one...

[Statilius the Epicurean]

Agree with most of the above, but it's misleading (and was a trait of orientalist western observers) to view Japanese opinion on the treaty as monolithically militarist. The Navy did insist on 70% but was overruled by the civilian government in Tokyo where, much like in London, there was an agreement among all parties that Japan had to restrain military spending and couldn't compete with the United States in a naval arms race. And in fact the pro-Treaty Faction (including officers like Admiral Yamamoto, who had studied in the US and were well aware of American industrial capacity) held the upper hand in the Navy until the 1930s, after the invasion of Manchuria made disarmament a dead letter.

And in the leadup to the Washington Conference the United States had put immense pressure on Britain to not renew the Anglo-Japanese alliance. One of the major goals of the Conference was to break any agreement between Britain and Japan that could threaten US interests in the Pacific. Of course Britain was desperately seeking an agreement on war debts with the United States and had no ability to refuse American demands on that point. Which is indeed why the Washington Naval Treaty marked not just the British Empire's acceptance that the United States was "just as powerful", but acquiescence to American hegemony and that even Britain's ability to conduct an independent foreign policy was constrained by US strategic demands.