What does it mean for an application to be "withdrawn"? If an asylum seeker gets a marriage/family/work visa and stops a now-unnecessary asylum process, is that a withdrawn application?
I suspect that "withdrawn" normally means the UK government has paid them to cancel their asylum request and leave the country (after which, admittedly, they might try again) but I don't have the figures to know how common that is.
Just to ground this discussion in some actual data, here are the figures from a 2019 news article[0]:
"It shows how, of the 80,813 applications that were refused or withdrawn between 2010 and 2016, only 29,659 individuals were removed, leaving 51,154 failed asylum seekers in the country from that seven-year period."
So it would be better to say that "only 37% of failed asylum seekers are deported". Also, I don't think that 51,154 people in seven years entering a country of 69 million really represents "unlimited migration".
[0] https://news.sky.com/story/fewer-than-half-of-failed-asylum-...