Degree calculation

Does anyone know how they calculate the degree classification (Roehampton University) psychology school.
I have been told that usually the first year doesn’t count towards your finial grade.

To quote from section 25, Award and classification of Bachelor’s Degrees, in Roehampton's Taught Degree Regulations 2023/23 (here) - my my added emphasis:
"(e) Save for the provisions of (f), (g), (h) (i) and (k), Bachelor’s Degrees are classified on the basis of the average of the percentage marks in the best 100 credits at Level 6 and the next best 100 credits at Levels 5–6 which the student has achieved at the University (i.e. not through credit transfer), irrespective of subject and with no weighting between levels, rounded to the nearest whole number. Each mark is weighted in the calculation according to the credit value of the module to which it applies. Where a student has achieved fewer than 200 credits at the University, the award will be classified on the basis of the average of the percentage marks in all of the credits at Levels 5–6 which the student has achieved at the University, irrespective of subject and with no weighting between levels, rounded to the nearest whole number. For students registered on level 5 in 2020/1 there is a variation to the algorithm as set out in Appendix B."
You can ignore the "provisions of (f), (g), (h) (i) and (k)", as they refer to "students who have completed an approved period of study abroad prior to September 2016", "Bachelor of Arts in Primary Education", "Bachelor of Fine Arts", "a Bachelor’s Degree having completed a top-up year after being awarded a Foundation Degree by the University", and "a student who is unable to complete their studies as a result of unforeseen exceptional medical or other circumstances".
So, you are correct that your first year (Level 4) doesn't count. What does count is the best 100 credits in third year, and then the next best 100 credits in second or third year.
They than take the average from the calculation from 25(e) above, rounded to the nearest whole number, and award you a First Class Honours for 70% or above, else a Second Class Honours (Upper Division) for 60% or above, else a Second Class Honours (Lower Division) for 50% or above, else a Third Class Honours for 40% or above.
Edited to add that there's a special rule if you're within 1% of a boundary, where they award you the next classification above if at least 60 Level 6 credits are at that higher classification.

To quote from section 25, Award and classification of Bachelor’s Degrees, in Roehampton's Taught Degree Regulations 2023/23 (here) - my my added emphasis:
"(e) Save for the provisions of (f), (g), (h) (i) and (k), Bachelor’s Degrees are classified on the basis of the average of the percentage marks in the best 100 credits at Level 6 and the next best 100 credits at Levels 5–6 which the student has achieved at the University (i.e. not through credit transfer), irrespective of subject and with no weighting between levels, rounded to the nearest whole number. Each mark is weighted in the calculation according to the credit value of the module to which it applies. Where a student has achieved fewer than 200 credits at the University, the award will be classified on the basis of the average of the percentage marks in all of the credits at Levels 5–6 which the student has achieved at the University, irrespective of subject and with no weighting between levels, rounded to the nearest whole number. For students registered on level 5 in 2020/1 there is a variation to the algorithm as set out in Appendix B."
You can ignore the "provisions of (f), (g), (h) (i) and (k)", as they refer to "students who have completed an approved period of study abroad prior to September 2016", "Bachelor of Arts in Primary Education", "Bachelor of Fine Arts", "a Bachelor’s Degree having completed a top-up year after being awarded a Foundation Degree by the University", and "a student who is unable to complete their studies as a result of unforeseen exceptional medical or other circumstances".
So, you are correct that your first year (Level 4) doesn't count. What does count is the best 100 credits in third year, and then the next best 100 credits in second or third year.
They than take the average from the calculation from 25(e) above, rounded to the nearest whole number, and award you a First Class Honours for 70% or above, else a Second Class Honours (Upper Division) for 60% or above, else a Second Class Honours (Lower Division) for 50% or above, else a Third Class Honours for 40% or above.
Edited to add that there's a special rule if you're within 1% of a boundary, where they award you the next classification above if at least 60 Level 6 credits are at that higher classification.

This seems a way of inflating degree classifications

Are you referring to the "if you're within 1% of a boundary" part? Many universities have a similar rule, to avoid a "cliff-edge" between 59.49% and 59.50% (for example).

No, just taking the average of the best 100 credits

It's a weighted average of the best 100 in third year and best 100 of the remaining modules in third and all of second years.

Does anyone know how they calculate the degree classification (Roehampton University) psychology school.
I have been told that usually the first year doesn’t count towards your finial grade.