The question here asks to "evaluate the effectiveness," so your answer should focus on discussing what elements of the Warnock Report and subsequent legislation were effective, but also which elements did not go far enough.
So you might consider:
1. The Warnock Report was effective in that it was a landmark. It was the first wide-ranging investigation of special educational needs (SEN) provisions in the UK and hoped to prepare children being educated in the SEN system to enter employment.
2. It was also effective because it marked a shift in how people who were "handicapped" were discussed and provided for. In the 1950s and 1960s, students were simply referred to as "handicapped" and the question was simply whether or not they could be educated. In the Warnock Report, this was dismissed as outdated: now, individual students were looked at in terms of their context, background and particular needs. This is in fact the origin of the term "special educational need" as is now widely used.
3. The Warnock Report also identified the fact that about one in five of all children will have a special educational need at one time or other—SEN children are not a separate category of "others."
4. The Warnock Report formed the basis for future development of these ideas. For example, the Education Act 1981 set out the idea that children's needs were all relative to each other, and that Local Education Authorities should be responsible for helping schools identify children with special needs.
5. Some drawbacks of the Warnock Report today are that it was written in a very different context from today, within which there was no national curriculum and schools were 90% state-run by Local Aeas. Now, it is argued that academies, which are state schools not controlled by Local Areas, tend to exclude students with SEN because this will cost them more to educate these students. So some of the provisions of the Warnock Report are not fit for purpose in the modern context.
The Disability Act 2001 had a different purpose from the Warnock Report. Its goal was to establish legal rights for disabled and SEN people in terms of their education and employment opportunities. So, while Warnock was invaluable in terms of changing how people were categorized and helping them access more individualized education, it did not offer legal protections. The 2001 act exists to make discrimination in terms of goods, services and education actively unlawful. Students now cannot be unjustly excluded from schools, colleges, youth services, etc. This act helps to compensate for some of the issues Warnock does not cover, but as noted above, the academization of education has provided schools with loopholes to get around this.