I think we shouldn't just leap to write off the USA as an ally, as a knee-jerk reaction to the current diplomatic situation.
consider that the US will still continue to staff and fund 13 military bases in our country, we are strategically important to US and will remain so.
also consider that Trump is one president who takes a position but at some stage he will not be president. Similarly Putin is one person, who is here at this point in history and his actions at some stage will cease. All things will pass.
Leaping to the conclusion that US is "no longer an ally" is short termist in the grand scheme. We definitely need to step up decisively and fast, in bolstering European security (that has been known for a very long time) - successive governments have been milking the peace dividend for far longer than was wise to do and we know we need to take affirmative action so in that regard Trump has done us a favour. But let's not write off US.
The US UK "special relationship" that leaders would have us believe, is a bit of a myth, it has been symbiotic and often cordial when needed, but there has also been a power imbalance. This is international relations for you. It's nuanced.
Thatcher and Reagan, Blair and Bush, they had personal rapport, but they are history now too.