Species Surveys (good for long-term monitoring of change/feeding into national datasets)
Stag Beetles / Hedgehogs / Dormice / Watervoles - https://ptes.org/get-involved/
Birds (advanced) - https://www.bto.org/our-science/projects/breeding-bird-survey
Butterflies - https://butterfly-conservation.org/butterflies/recording-and-monitoring
Plants - https://www.npms.org.uk/
Baseline Survey Methods
UK Habs - a new-fangled and widely accepted method. Not too challenging, but requires some knowledge of plants species. Requires you to create a free account to access documentation. Also has a pay-to-use app for Android and iOS. Will also give you a grade of the quality of the habitat. Feeds directly into Biodiversity Net Gains*. Probably the one I'd recommend - https://ukhab.org/
Phase 1 - the absolute bog-standard. Very easy, but doesn't tell you about habitat quality. Extended Phase 1 will give an indication of value, but is still rather tangential compared to PEA and UKHabs - https://data.jncc.gov.uk/data/9578d07b-e018-4c66-9c1b-47110f14df2a/Handbook-Phase1-HabitatSurvey-Revised-2016.pdf
Preliminary Ecological Appraisal - The professional standard for having a gander at habitats. Really quite advanced and may not be what you want! - https://cieem.net/wp-content/uploads/2019/02/Guidelines-for-Preliminary-Ecological-Appraisal-Jan2018-1.pdf