Does the open outer door of my porch give bailiffs access through the locked front door?

No

The bailiff can enter by turning the handle to any door, to any usual means by which entry is gained to the premises (for example, a loading bay to premises where a trade or business is carried on).^[1]

If the key is in the lock, the bailiff may enter by turning the key.^[2][3]

If the front door inside the porch is locked, the bailiff only has a right to break entry in very rare circumstances, and it's seldom practiced.^[4]

If the debtor keeps goods inside the porch and the porch door is unlocked, but the inside front door is locked, the bailiff may only take control of those goods inside the porch.^[5]

^{[1] Regulation 20(a) of the Taking Control of Goods Regulations 2013
[2] Ryan v Shilcock [1851] 7 Exch 72
[3] See Can a bailiff enter my house
[4] See Bailiff threatened you with a locksmith
[5] Paragraph 10 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007}