How would you deploy such a weapon? Presumably that's what RAF Fylingdales is for?
Fylingdales is used to detect ICBM flight paths and track orbital objects. Neither of which would be a factor in the of a tactical nuke in Ukraine.
Russia possesses battlefield nukes - artillery and rocket - as well as SRBM, such as the Iskander. It was intelligence suggesting the Iskander deployed to Kaliningrad were equipped with nuclear warheads threatening Europe which led to the US withdrawing from the INF treaty.
They also have air launched ASM with nuclear warheads, such as the Kent and the Kickback, carried by the Bear and Blackjack. Though with their conventional versions having a reported success rate below 50% I’m not sure they’d risk using one.
None of which would be detectable by Fylingdales.