The two sentences which need to be connected are: "We all considered it great fun." and "I regret our youthful thoughtlessness."
You can tell something is missing between these sentences because they are expressing radically opposed ideas. The first one is very positive and optimistic, and the second one is full of regret. Therefore, they should be linked by a conjunction expressing opposition, such as <em>but</em>, <em>however</em>, or <em>yet</em>.
You could even add a clause which explicits the time difference (then versus now), to mark the contrast even more. An example would be: "We all considered it great fun. <u>Yet, now that I have grown up,</u> I regret our youthful thoughtlessness."
The two sentences which need to be connected are: "English cooks seemed unable to prepare an appetizing vegetable dish." and "Vegetables were very abundant at the time and were grown in the gardens of both rich and poor."
Here again, the two sentences seem to raise a paradox: why would the cooks be so unexperienced with vegetables if they had plenty at their disposal? One way to connect these statements would be to use a linkword which expresses this apparent contradiction. You could insert <em>surprisingly</em> or <em>although</em>, for example.
If you want to add a full clause or sentence, you could point out this question, like so: "English cooks seemed unable to prepare an appetizing vegetable dish. <u>This is not due to a lack of such produce, however: </u>vegetables were very abundant at the time and were grown in the gardens of both rich and poor."