@linknet
The roaming indicator will never disappear from your display unless you do not have a Dutch IMSI/local number and you are in the Netherlands using the Roaming IMSI.
The reason for this is the following:
Each network on this planet has at least one unique ID consisting of a
mobile country code (MCC) and mobile network code (MNC). E.g. in the UK O2 use 234-10 and 234-11 for their network.
Also each SIM card holds a data field called Home Public Land Mobile Network (HPLMN) where the MCC/MNC of the issuing operator is stored.
What your phone does is simply comparing the MCC/MNC of the current network with that one stored in the HPLMN field in your SIM card. If both numbers match then the phone assumes that you are on your home network, while if they mismatch it displays the roaming icon.
Now if you use the British IMSI the HPLMN field will be set to 234-26, the MCC/MNC-tuple of lycamobile, the company behind toggle. As an "full MVNO" lycamobile do operate their own core network but do not have a radio access network. Instead they let their customers use the O2 network in the UK, which identifies as 234-10 or 234-11. Since the HPLMN field of the British IMSI and the MCC/MNC of the O2 network mismatch, your phone will even show the roaming icon while you are in the UK.
If you use the Roaming IMSI which actually comes from Vodafone Netherlands the HPLMN field will be set to Vodafone NL's MCC/MNC (204-04) and your phone will consequently consider any other network than Vodafone NL as roaming network. Hence the Netherlands are the only country where your phone would not "roam" in a technical sense. This would change as soon as you activated a Dutch IMSI/local number for your SIM card because then you would still use the Dutch Vodafone network but the Dutch IMSI would change the HPLMN to 204-09 (lycamobile Netherlands) so again there would be a mismatch of HPLMN and the current network's MCC/MNC-tuple.
I have seen some Android phones that obviously know which networks all the MVNOs use for domestic coverage and hence suppress the roaming indication in the SIM card's home country, but especially older and cheaper phones like your Samsung do not offer such advanced network detection. Therefore you will often see SIM cards from so-called full MVNOs who use their own MCC/MNC-tuples to be "roaming" on their native networks.
EDIT: Just discovered
here that 3GPP standards now allow for a Equivalent HPLMN list on SIM cards which can specify more than one network as home network. The question remains if toggle makes use of this feature and which phones extract and use this information from the SIM Card.
After all this lengthy explanation I have a simple answer for you: While you are in the UK you must be using the British IMSI because the Roaming IMSI is blocked for roaming on British networks.
Btw toggle appear to also block roaming for other countries as soon as you obtain a local IMSI/local number.