GBP/USD has failed at the 1.3363/1.3392 resistance area - the July high and 50 percent of the 2018-2019 fall - as optimism over avoiding a no-deal Brexit is replaced by concern at the lack of a viable deal.
GBP/JPY and GBP/AUD have also stalled at major levels, and a break in either cross could potentially be the canary in the coalmine for a GBP/USD move.
GBP/JPY failed at the 148.25 horizontal weekly cloud top, just below September, October and November range resistance at 149.50/70, and currently trades around 147.30.
Should 150.00 break, it would be a strong signal that 1.3400 in cable would be vulnerable, whereas a close below the 145.72 weekly cloud would suggest a GBP/USD top is in place.
GBP/AUD failed at the October 1.8734 high in late February/early March and a close above 1.8750 would be a positive GBP signal.
A close below 1.8142, 38.2 percent of the December-March rise, would be bearish, but looks unlikely for now, barring a no-deal Brexit or major AUD catalyst.
Though the crosses only occasionally drive cable, it is worth keeping an eye on the major levels as they can influence moves.