Someone can probably answer that question better than I could., but let us give a try nonetheless.
Let us consider that we have an independent market between the US and the UK and that the values of their respective money is only depending on that market. That is no influence outside those two countries will be considered.
At this market, the exchange rate between the pounds and the dollars is a consequence of the buying and selling of dollars against pounds. If we further simplify, supposing that no government, currency speculation, or otherwise take place, we can reduce the exchange rate to be a consequence of the commercial balance between those two countries. The more the US import, the more the pound/dollar ratio increases. And the more they export, the more the ratio decreases.
Now, we consider that the prices in the UK rises as an effect of the inflation. The US inflation on the same period is 0.
At first, the exchange rate was unchanged. Export goods are kept at the same price (no inflation in the US and constant exchange rate). However import goods are more expensive due to the inflation. This favours the US export in comparison to its import. As a consequence, there is a higher demand of US goods in the UK (cheaper) and a lower demand of UK goods in the US.
The effect is a fall of the pound in comparison to the dollar.
It can be seen in another way, the acquisition power of the pound has lowered due to the inflation, and the exchange rate reflects that against the non-inflated dollar.
As a consequence, the prices of the US exported goods will rise, whereas the prices of the UK exported goods will lower. This will re-establish the variation of demand and offer caused by the inflation to what it was before, but with a new echange rate, reflecting the new acquisition power of the pound.
This is a theoretical illustration of the self-regulation of the market.