IPSTAGs are the Nominet identification for each registrar. So Tsohost has its own IPSTAG of UKWEBHOSTING.
Transferring a UK domain between providers is just a case of switching that IPSTAG to the new provider.
This is instant, as in, there is no delay whatsoever in changing the IPSTAG from the old provider to the new one.
However, when you request an IPSTAG change from your previous provider, they may not pass that request immediately to Nominet. Some take longer, and some much longer. A partial delay for sanity checking would be legitimate.
The process of changing an IPSTAG is also completely free, and Nominet charge no money for it. Therefore if you are being charged for the privelige of changing an IPSTAG then you are being ripped off by a dishonest provider. If you can change your nameservers automatically and for free through a provider's systems then it should be equally easy for you to change your IPSTAG through their systems.