[mv] Setting locale via a form

I am trying to set locale via a form so that a user can select their preferred currency from a drop down menu. I've tried setting the scratch variable mv_currency, and tried using mv_click as shown below but to no avail:-

      <FORM METHOD="POST" ACTION="[process-target]">
       <INPUT TYPE=hidden NAME="mv_todo" VALUE="return">
       <INPUT TYPE=hidden NAME=mv_nextpage VALUE="@@MV_PAGE@@">
       <TABLE WIDTH="130" CELLPADDING="2">
        <TR>
         <TD>
          <SPAN CLASS="navtitle">CURRENCY</SPAN><BR>
          <SELECT NAME="mv_click" onChange="this.form.submit()">
          <OPTION VALUE="Cu_USD"> US Dollars
          <OPTION VALUE="Cu_GBP"> UK Pounds
         </SELECT>
         </TD>
        </TR>
       </TABLE>
      </FORM>

[set Cu_USD]
[setlocale locale=USD currency=USD persist=1]
[/set]

[set Cu_GBP]
[setlocale locale=GBP currency=GBP persist=1]
[/set]

This must be possible, but I am at a complete loss as to why I cannot do it - it seems that you can only set locale via a direct setlocale tag or a link. Has anyone out there managed to do this via a form as shown above? It would seem to me very sensible for a user to be able to select a currency from a drop down list, especially if you have a whole range of currencies as we do here in Europe. I would very much appreciate it anyone could point me in the right direction.

Thanks in advance