Requirements are constantly changing. I suggest before traveling to search the UK and NL government sites. Just google UK covid entry requirements and the same for the NL.
Our current understanding is that are traveling from a Green country (India is currently Green) and you have proof of being vaccinated with an approved vaccine and arrive after 4AM Monday Oct 4, you do not have to have a negative test to board your flight; however, you must pre-book a 2nd day Covid test (delivered to your hotel or booked at the arriving airport), You must also complete a Passenger Locater Form within 48 hours of your departing flight which requires a confirmation code of the testing company that you have pre-booked the 2nd day test. We are unsure about the in transit rules if you are flying in and out within the 2 days. Check the UK Gov site for current rules and which countries are Red and Green. Rules for arrival from a Red country are different
As for the NL 48 hour pre-board negative test, you can get one at the arriving airport (pre-booking an appt is highly recommended and you will probably have to spend a night near the airport) or in major UK cities. We would recommend a same day test as the 48 hour window is not much.
If you stay near Marble Arch, we found a same day testing site in Chelsea ( St Luke's Christ Church Lab, Sydney St. SW# 6NH). Best I can remember it was 134GBP.