A very confusing issue, with a very clear explanation.
Understanding The Policy
You can read the solution below, however, it is, paramount to understand what the policy is, to avoid any future issues.
When you target a country, for example, United Kingdom then the currency you submit must match the currency on the product landing page. In this case GBP.
The reason is that when a user is not located in the United Kingdom but does see an ad, then the price and currency in the ad must match the product landing page.
While currently there is no way for a user to see the ad outside of the target country, you still need to follow this policy. This may be due to a future plan to allow your ads to display worldwide or it is a law requirement in a majority of countries. Or even so that Google can validate the prices with manual reviews. For example, Google may have a team in India to check the ad price compared to the landing page price.
Regardless of the reason, and how backward you may think this is if you want to remove or avoid an account suspension, follow the policies.
Manual and automated reviews are done all the time to ensure merchants do not violate policies.
Most Common Reason
Auto Currency Conversion
The most common issue is that you have enabled automatic currency conversions. While localization is indeed key to success, if you want to advertise on Google Shopping, the price and currency submitted in the feed must match the currency and price on the landing page. So any server-side or client-side price changes are not allowed. Instead, you are allowed to have a manual drop-down to change country or currency.
In this case, remove the automatic currency conversion system you have integrated.
You may have set up structured data with the wrong currency. You can use Google’s structured data testing tool to check the product data.
In this case, update your structured data.
If you want to advertise in multiple different countries, you can choose to submit a dedicated feed, meaning you submit the same language and currency in the data feed as the website.
Or you use the same default data feed and in Google Merchant Center > Products > Feeds > click on your feed > Settings, add additional countries.
Remember I said localization is key to success, if you use automatic currency conversion within Google Merchant Center, you will not get as many eligible impressions as a dedicated feed.