First of all, let's consider "It was all a bit." This is very casual or informal English; it is usually said only face to face or written in email. Also, it is used in narrative communications (a report, account, or story you present) that is given upon reflection. Its use implies that. It's form comes from reflecting by memory on some specific (particular, mentioned) event. So you are evaluating an experience you have already named or signified; basically sizing it up. You use this form in order to orient a reader or introduce your opinion about some particular (specifically in the mind at the moment) personal experience. "It" means something already indicated or spelled out in name. "It was last minute" is a shortened, casual-evaluation phrase in English. "Done at the" is dropped out from it. The base form in the mind is "It (something already referred to) was done or performed at the last minute"; also, "It was on the spot" ; also, done without preparation. It is shortened in the sense that there is no verb before it (as would given be in the parallel form "It was done at the last minute"), or noun placed after it ("It was a last minute performance/activity"). Dropping out a verb and article in description makes it ultra-casual or slang. Another example, "The statue was all make-shift." Formally with its implied logic complete said, it means "The statue was all erected/made without adequate or sufficient preparation." The "all" can be taken/inferred to mean either "all of the statue" or "all of the activity." Which one it is is left ambiguous/unclear in casual conversation. "A bit" is added in, also, resulting in "It was all bit last minute." That means the experience was "somewhat/a little/somewhat/sort of "(indefinite or imprecise in its amount or quality) make-shift. This is slang-like because dangling an adjectival phrase at the end of a sentence like this is not grammatically proper or appropriate in traditional formal discourse.

At the Last minute is the time just before time frame that something must be done or decided ,otherwise it cannot be done after that , or the ​latest ​possible ​opportunity for doing something It was all a bit last minute > meaning you have decided to do something just when the opportunity is about to be gone or cannot be done anymore. It can also mean that you have decided on something at the last possible moment They only told me at the last ​minute that they couldn't come.( the last opportunity in time to say they cannot go) Why do you always ​leave everything until the last ​minute? (doing something in the time period that is just before its too late )