As falcon900 and others say, there are certain things that require a certain physical presence. If this trip was to discuss important issues, then the UK needed to impress. The cost of this trip seem eye-watering to us mere mortals, but we are probably not very conversant with the sums involved in running a country.
The refurbishment of St Pancras railway station in London was apparently £800 million. HS2 will cost ~ £80 billion or more, which makes £500 thousand just small change. Having said that, it does seem excessive, and one wonders why they couldn't use a scheduled BA flight for example - assuming the service is running? Perhaps standard passenger aircraft security is not sufficient for Government ministers?