API to query info about units tariff band now and get upcoming band change times

I'd like to have some kind of ultra-ultra-lightweight API that allows a client to query

(i) which units-charging tariff band we are in at the current time together with the current download charging rate, and

(ii) the times (unix time) of upcoming tariff band changes.

It would be nice if there were a choice of protocols and formats for querying and publishing this info. Your ideas will be better than mine, but one idea could be queries and responses in UDP packets, very low-cost, with binary TLV records, so no parsing costs and small code. I believe queries would need to have parameters with info about the client or the line because the answer depends on the geographical location of the line seeing as AA bank holidays vary according to nation. It would be really nice if there could be a choice of various completely different types of identifying information parameters supplied.

A second thing: In an ideal world, a Firebrick would be able to exploit this API.