Source is here: https://yougov.co.uk/topics/politics/survey-results/daily/2025/02/20/c33bd/1
That source allows you to see the results according to political affiliation. Pluralities of supporters of the Conservatives, Labour, and Lib Dems think that supporting Ukraine is more important.
The exception is Reform UK, whose supporters think that good relations with the USA are more important than supporting Ukraine.
Why is there a “Neither” option? I mean in practice “Neither” could mean to strike a perfect 50/50 balance. Or it could mean to antagonize both of them. Or it could mean “i don’t really care”. So what makes “Neither” a separate and relevant category to “Don’t know”, which could also mean “i don’t know how to prioritize the two” or “i don’t really care” or “what is an US and is it a newer model or older than the Ukraine?”
“Neither” implies they have an opinion on the relative prioritisation of the US and Ukraine by the UK, and believe that both should be prioritised to a similar degree:
“Don’t know” implies they do not feel they know enough about the relationships the UK has with either country to form an opinion.
But yes, there is some fuzziness, as anyone who does not care is more likely to choose “neither” as there is no “either” option, yet many of this type of respondent will have picked “don’t know”.