Most claims are accepted because of the shear fact that they're here after crossing. If we put up a border post in France then it'll become different as those people are not here and so their case weakens.... So either way as it stands they'd still attempt the crossing regardless.
Other countries in Europe take people according to arrivals and that's dominated completely by Germany. The reason for that though is anyone can literally walk into Germany and claim asylum. The issue here is unlike elsewhere there are 30 miles of water between the UK and continental Europe.
We could make it easier for cases to be accepted if you're not in the country, for example in Calais but what criteria do you attach to that and who is going to pay for and control everything for processing because the French certainly won't?
If someone turns up to a UK border post in Calais with no passport, no background and we have no idea who they are then do you just accept their word and provide housing, food, healthcare and transport to the UK? The fact you cannot reject or send these type of people back once here is why they make the crossing.