Not based in the UK - but I've got experience sending stuff from mainland Europe to the USA. Just over 2 weeks for a (tracked, first class/priority) letter was typical prior to the current Virus. You'd always see the tracking tell you it got to the US within a day of dropping it off at the post office - followed by customs clearance quite quickly - followed by 2 weeks of nothing until it suddenly arrived at the destination city.
But a recent letter: departed Europe the day after dispatch... next tracking event was arrival in the US almost 2 weeks later. That could mean it waited almost 2 weeks to actually be put on a plane, or they're being slow at processing on arrival on the US (seems more likely - the arrival point in the US happens to be NYC where I imagine things are chaotic). It's still nowhere near the destination city.