 Here is the Python code that solves a 2D magnetostatic problem for a single coil using the bluemira library and FEniCS:

```python
from bluemira import *
from bluemira.components import *
from bluemira.geometry import *
from fenics import *

# Define parameters
coil_radius = 0.1
coil_height = 0.05
enclosure_radius = 0.2
enclosure_height = 0.2
mesh_size = 0.001
num_ois = 10

# Create coil and enclosure
coil = SolidCircularCylinder(radius=coil_radius, height=coil_height)
enclosure = SolidCircularCylinder(radius=enclosure_radius, height=enclosure_height)

# Set mesh options
coil_mesh_options = __create_dict_for_mesh_size(mesh_size)
enclosure_mesh_options = __create_dict_for_mesh_size(mesh_size)

# Create components
universe = Universe(components=[coil, enclosure])

# Create mesh and convert for FEniCS
universe.create_mesh(mesh_options=[coil_mesh_options, enclosure_mesh_options])
universe.convert_mesh_to_fenics()

# Instantiate magnetostatic solver
solver = transport_code_solver(universe, systems_code_solver('magnetostatic'))

# Define source term
source_term = Expression(0, sin(2 * pi * x[1]), degree=1, element=FiniteElement('Lagrange', triangle, 1))

# Plot source term
plot(source_term, title='Source Term')

# Solve the magnetostatic problem
solver.solve()

# Calculate magnetic field
H = solver.get_solution('H')

# Theoretical value for a single coil (Ampere's Law)
r = sqrt(x[0]**2 + x[1]**2)
z = x[1]
theoretical_H = (2 * pi * coil_current) / (2 * pi * r) * (z - coil_height / 2)

# Compare calculated and theoretical values along the z-axis
z_values = linspace(0, enclosure_height, 100)
calculated_H_z = H.vector().get_local().reshape((len(z_values), 1))[:, 0]
theoretical_H_z = (2 * pi * coil_current) / (2 * pi) * (z_values - coil_height / 2)
diff_H_z = calculated_H_z - theoretical_H_z
plot(z_values, diff_H_z, title='Difference along z-axis')

# Compare calculated and theoretical values at a radial path at a certain z-offset
z_offset = 0.05
r_values = linspace(0, coil_radius, 100)
calculated_H_r = H.vector().get_local().reshape((len(r_values), len(z_values)))[:, 50]
theoretical_H_r = (2 * pi * coil_current) / (2 * pi) * (z_offset - coil_height / 2) * r_values
diff_H_r = calculated_H_r - theoretical_H_r
plot(r_values, diff_H_r, title='Difference at a radial path')
```

This code assumes that you have already set the `coil_current` variable and that the bluemira library and FEniCS are properly installed.