You need to find bonafide seedstock (good luck!) with a type III chemotype (these are seeds that are demonstrably known to produce high CBD, low THC chemical profiles in the resulting plant). I know that PCR testing of plant DNA can detect CBD chemotypes before you have flowers (important if you want to avoid no-CBD phenotypes that can be expected to have other, illegal cannabinoids in greater concentrations). See the most recent hash church for information on an affordable version of this sort of testing equipment. This will not tell you whether you have a Type II or Type III chemotype though, the distinction being that Type II tends to contain significant THC as well as CBD.
However, if you grow your own Type III plant, you still run the risk by chance that even a type III chemotype plant may produce more than the legally allowed level of THC in the plant (IIRC the UK requires below .03% of THC - I may be wrong here). The amount of THC in resin varies between plants over time, even with the same seed supplier. This is organic material, it does vary. Essentially, do not think you are protected from legal problems in prohibition land just because you are growing high CBD flower.
Also, your assumption that sensimilla is always the most resinous material can be found to be wrong in some cases. IME, intersex plants in some varieties/phenotypes can actually produce the highest resin content, more than seedless females. Skunkman Sam has mentioned this in the past too on hash church, in some cases intersex plants get better resin coverage than females. This will vary depending on what you have. What is most important is to focus on varieties and phenotypes that are known to produce large quantities of resin, which have the desired CBD content. Varieties that are known for CBD but yield low % of CBD and low overall quantities of resin are not going to be the most useful medically.
In your case, you may prefer to purchase the abundantly available pure CBD isolate from vendors in Europe (Isodiol is a tried and true manufacturer!). This is 99-100% crystalline CBD. Buy some blue river terpenes (DO NOT BUY OTHER TERPS AS I CANNOT SPEAK TO WHETHER THESE ARE CANNABIS DERIVED OR WHOLE TERP PROFILES - IMPORTANT FOR RATIOS - YOU MAY BE TAKING UNNECESSARY RISKS).
Yes, these are expensive. However, you can be sure that you have what you need.
Mix 19:1 parts (CBD isolate:blue river terps) of your CBD and terps to create a formulation containing 95% CBD and 5% total terpene concentration. This will be comparable to rosin or solvent extracts made from Type III high CBD flower, with no risk of THC in your material. This should keep you out of trouble with the law which I assume is a goal if you consume CBD hemp derived material in the UK?
I suggest terp profiles from high CBD varieties for the best ensemble effect results. High myrcene terp profiles are also recommended for this.