There have been a couple of events here basically.
First is GBP/US$
This was in the $1.80 - $2 range between about 2004 and 2008.
During this time $1 = around 9,000 IDR
So this gave an exchange rate of around 16,000-18,000
The pound then fell to just above the $1.40 mark in early 2009 before recovering to a band around $1.50-$1.70 between 2009 and 2014, after which time it fell to around $1.45 just before the Brexit vote in June. The Brexit vote brought a more-or-less immediate 10% haircut for the pound and it's remained about $1.30 since then.
The 9,000 mark at which the rupiah was defended by Bank Indonesia was abandoned around 2012, and there was a more-or-less continuous steady decline to around 14,700 in September 2015. This was the rupiah's low point as the Bank re-established a target of 13,000 IDR which knocked the rate down to 13,700 fairly swiftly and in 2016 actually down to more-or-less the 13,000 mark.
In the post-Brexit period the rupiah has gained a relatively insignificant 2% against the dollar, but over the longer term a strengthening of the rupiah of at least 10% and a weakening of the pound by at least the same figure means that those holding sterling are looking at cost increases of 20-25% in Indonesia in the recent past.
Yes, £1 = 16,800 was seen only in August of 2013, that's 3 years ago, but since that time a lot of things have got significantly more expensive in rupiah terms.