To be able to search using UK formated values you will need to update the value in the DataAssist Search server behavior for your date elements. You can update them like this:
You would need to ensure that it is your date field that is referenced. As for searching by year only the best way to do this would be to have a select list with the labels set to years, but the values set to January 1st for that year, like this:
<select name="select1" id="select1">
If you need any help with formating the date to work with UK dates post back with your results page and let me know what the date fields are called.