Ashford is only 40 minutes away from London King's Cross so you could stay in either place easily. The night of the wedding it would probably be easiest to stay locally, but you could catch a late train back to London if you wanted. We are hosting a few events the week of the wedding, so check the Schedule tab in case that influences your decision.
Your presence is enough for us but if you'd like to give a little something we have a few items posted here including a few of our favourite charities that are always looking for donations: https://prezola.com/buy/view/268797