You can’t tell why GBP/USD rallied just by looking at only 1 currency pair.
So what you can do is to reference it to other Cross Currency pairs.
If the GBP/AUD is strong as well, then GBP/USD’s rally is due to a strong Pound.
But if the GBP/AUD is weak, then GBP/USD’s rally is due to a weak US dollar.
You can also check against any other Pound Cross Currency pairs.
If they’re in an uptrend – Pound is likely to be strong.
If they’re in a downtrend – Pound is likely to be weak.