From London you can fly to Glasgow and then car + ferry to most of the islands. There are also flights to many of the islands from Glasgow and a few from other airports , such as Aberdeen. These will be on small planes, so not for the nervous. Lewis and the connected Harris also have fantastic beaches. If you get there, I would recommend trying to get a boat trip to the Shiant Isles (uninhabited) which are a few miles south east of Harris. There are a multitude of sea birds here, with over 100,000 pairs of puffins and guillemots, razorbills, little auks, skuas, kittiwakes, fulmars, etc, plus the white tailed eagle. A true bucket list location. I have been there twice.
There are a lot of ferry links between the Outer Hebrides, so if you have time, you could take in several. Mull is also interesting, being known for white tailed eagles and from there you can get on a trip to Staffa (Fingal’s Cave) that has a large number of puffins. From boats you will have the chance to see more sea birds, plus dolphins, basking sharks, Orcas and minke whales. A lot of the birds can also be seen from many places on the west coast of Scotland.