Can bailiffs leave an enforcement notice exposed outside the entrance of my secure flat?

No

Bailiffs may only give the Notice of Enforcement either by post addressed to the debtor at his place of residence,^[1] by electronic communication,^[2] by delivery through the letterbox,^[3] and where there is no letterbox, place it where it is likely to come to the debtor's attention.^[4]

^{[1] Regulation 8(1)(a) of the Taking Control of Goods Regulations 2013
[2] Regulation 8(1)(b) of the Taking Control of Goods Regulations 2013
[3] Regulation 8(1)(c) of the Taking Control of Goods Regulations 2013
[4] Regulation 8(1)(d) of the Taking Control of Goods Regulations 2013}