From what I've read, about 1/3 of all asylum seekers over the last 7 years arrived via small boat crossings.

Looking forward though, about 90% of those arriving in small boat crossings are currently going on to seek asylum and the average annual cost of supporting an asylum seeker during their claim has risen to an estimated £41k, so for ~30k arrivals this year, the financial cost of not processing these claims promptly could increase that overall annual bill further still.

Also, in the first year of processing, costs may be drawn from the overseas aid budget (which was recently shrunk). This results in possibly 1/5 of the overseas aid budget being used for costs associated with processing asylum claims, which perhaps doesn't match most people's expectations as to what overseas aid should be used for.

I think that's why even though the number of people involved in these crossings is small compared to net migration, it has a big financial impact.