This should be relatively simple in a server-side implementation, the most simple option would be to create an activity that returns a JSON offer with something like....
{currency: "£"}
You would provide an mbox name as part of your server-side request to Target, and then your web application would parse the response and handle replacing all of the $ symbols.
You could also achieve this client-side but the ease of implementation will depend a bit on how easy it is to identify all of the currency symbols on the page. For example, if all of your currency symbols have a class of "currency", then you could add a custom code modification in the Visual Editor, something like......
document.querySelectorAll('.currency').innerText = "£"