For standard checkout at the UK PayPal site use the following at the beginning of your checkout form:
<form name="checkout_PP_LI" method="POST" action="https://www.PayPal.com/uk/cgi-bin/webscr">
To show amounts in pounds, put the following line in your checkout form:
<input type="hidden" name="currency_code" value="GBP">
HTH,
Simon