5W30 is not really available in the UK as a full synthetic, either use 0W40 or 10W40.
If you are doing long runs and service your car more than once a year then semi-synthetic would be fine. You could even use mineral oil if you change it every 5k miles.
Lexus only state the specifications that need to be met, which are in the owners handbook. Any oil that meets those specs can be used.
Most Lexus dealers use fully synthetic oil but some also use semi-synthetic.