I don't know whether US Customs allows you to declare "Goods to follow" for your personal exemption and how that applies to goods that might otherwise be subject to tariffs.
I don't know whether the tariffs has resulted in greater inspection and assessment of goods brought in by travellers and returning citizens.
Normally I would assume that shipments entering the UK would be subject to UK taxes on import.
https://www.gov.uk/import-goods-into-uk
I don't know if the UK Customs services at the points of exit will refund VAT paid on import (of parcels) as they might for UK goods bought for export.
I know if you ship to Canada you pay the tax plus fee on import, but can claim a refund if the goods are reexported. I don't think the fee is refundable though.

Regards

Mike C